diff --git a/docs/codeql/support/reusables/frameworks.rst b/docs/codeql/support/reusables/frameworks.rst
index be9949aad77f..3e169a0949bf 100644
--- a/docs/codeql/support/reusables/frameworks.rst
+++ b/docs/codeql/support/reusables/frameworks.rst
@@ -155,6 +155,7 @@ Python built-in support
    Django, Web framework
    Flask, Web framework
    Tornado, Web framework
+   Twisted, Web framework
    PyYAML, Serialization
    dill, Serialization
    simplejson, Serialization
diff --git a/python/change-notes/2021-06-08-twisted-add-modeling.md b/python/change-notes/2021-06-08-twisted-add-modeling.md
new file mode 100644
index 000000000000..364b8c027078
--- /dev/null
+++ b/python/change-notes/2021-06-08-twisted-add-modeling.md
@@ -0,0 +1,2 @@
+lgtm,codescanning
+* Added modeling of sources/sinks when using `twisted` to create web servers.
diff --git a/python/ql/src/semmle/python/Frameworks.qll b/python/ql/src/semmle/python/Frameworks.qll
index 3115c3ffac65..ecbf4eb6c868 100644
--- a/python/ql/src/semmle/python/Frameworks.qll
+++ b/python/ql/src/semmle/python/Frameworks.qll
@@ -19,5 +19,6 @@ private import semmle.python.frameworks.PyMySQL
 private import semmle.python.frameworks.Simplejson
 private import semmle.python.frameworks.Stdlib
 private import semmle.python.frameworks.Tornado
+private import semmle.python.frameworks.Twisted
 private import semmle.python.frameworks.Ujson
 private import semmle.python.frameworks.Yaml
diff --git a/python/ql/src/semmle/python/frameworks/Twisted.qll b/python/ql/src/semmle/python/frameworks/Twisted.qll
new file mode 100644
index 000000000000..5b014597157d
--- /dev/null
+++ b/python/ql/src/semmle/python/frameworks/Twisted.qll
@@ -0,0 +1,250 @@
+/**
+ * Provides classes modeling security-relevant aspects of the `twisted` PyPI package.
+ * See https://twistedmatrix.com/.
+ */
+
+private import python
+private import semmle.python.dataflow.new.DataFlow
+private import semmle.python.dataflow.new.RemoteFlowSources
+private import semmle.python.dataflow.new.TaintTracking
+private import semmle.python.Concepts
+private import semmle.python.ApiGraphs
+
+/**
+ * Provides models for the `twisted` PyPI package.
+ * See https://twistedmatrix.com/.
+ */
+private module Twisted {
+  // ---------------------------------------------------------------------------
+  // request handler modeling
+  // ---------------------------------------------------------------------------
+  /**
+   * A class that is a subclass of `twisted.web.resource.Resource`, thereby
+   * being able to handle HTTP requests.
+   *
+   * See https://twistedmatrix.com/documents/21.2.0/api/twisted.web.resource.Resource.html
+   */
+  class TwistedResourceSubclass extends Class {
+    TwistedResourceSubclass() {
+      this.getABase() =
+        API::moduleImport("twisted")
+            .getMember("web")
+            .getMember("resource")
+            .getMember("Resource")
+            .getASubclass*()
+            .getAUse()
+            .asExpr()
+    }
+
+    /** Gets a function that could handle incoming requests, if any. */
+    Function getARequestHandler() {
+      // TODO: This doesn't handle attribute assignment. Should be OK, but analysis is not as complete as with
+      // points-to and `.lookup`, which would handle `post = my_post_handler` inside class def
+      result = this.getAMethod() and
+      exists(getRequestParamIndex(result.getName()))
+    }
+  }
+
+  /**
+   * Gets the index the request parameter is supposed to be at for the method named
+   * `methodName` in a `twisted.web.resource.Resource` subclass.
+   */
+  bindingset[methodName]
+  private int getRequestParamIndex(string methodName) {
+    methodName.matches("render_%") and result = 1
+    or
+    methodName in ["render", "listDynamicEntities", "getChildForRequest"] and result = 1
+    or
+    methodName = ["getDynamicEntity", "getChild", "getChildWithDefault"] and result = 2
+  }
+
+  /** A method that handles incoming requests, on a `twisted.web.resource.Resource` subclass. */
+  class TwistedResourceRequestHandler extends HTTP::Server::RequestHandler::Range {
+    TwistedResourceRequestHandler() { this = any(TwistedResourceSubclass cls).getARequestHandler() }
+
+    Parameter getRequestParameter() { result = this.getArg(getRequestParamIndex(this.getName())) }
+
+    override Parameter getARoutedParameter() { none() }
+
+    override string getFramework() { result = "twisted" }
+  }
+
+  /**
+   * A "render" method on a `twisted.web.resource.Resource` subclass, whose return value
+   * is written as the body of the HTTP response.
+   */
+  class TwistedResourceRenderMethod extends TwistedResourceRequestHandler {
+    TwistedResourceRenderMethod() {
+      this.getName() = "render" or this.getName().matches("render_%")
+    }
+  }
+
+  // ---------------------------------------------------------------------------
+  // request modeling
+  // ---------------------------------------------------------------------------
+  /**
+   * Provides models for the `twisted.web.server.Request` class
+   *
+   * See https://twistedmatrix.com/documents/21.2.0/api/twisted.web.server.Request.html
+   */
+  module Request {
+    /**
+     * A source of instances of `twisted.web.server.Request`, extend this class to model new instances.
+     *
+     * This can include instantiations of the class, return values from function
+     * calls, or a special parameter that will be set when functions are called by an external
+     * library.
+     *
+     * Use `Request::instance()` predicate to get
+     * references to instances of `twisted.web.server.Request`.
+     */
+    abstract class InstanceSource extends DataFlow::LocalSourceNode { }
+
+    /** Gets a reference to an instance of `twisted.web.server.Request`. */
+    private DataFlow::LocalSourceNode instance(DataFlow::TypeTracker t) {
+      t.start() and
+      result instanceof InstanceSource
+      or
+      exists(DataFlow::TypeTracker t2 | result = instance(t2).track(t2, t))
+    }
+
+    /** Gets a reference to an instance of `twisted.web.server.Request`. */
+    DataFlow::Node instance() { instance(DataFlow::TypeTracker::end()).flowsTo(result) }
+  }
+
+  /**
+   * A parameter that will receive a `twisted.web.server.Request` instance,
+   * when a twisted request handler is called.
+   */
+  class TwistedResourceRequestHandlerRequestParam extends RemoteFlowSource::Range,
+    Request::InstanceSource, DataFlow::ParameterNode {
+    TwistedResourceRequestHandlerRequestParam() {
+      this.getParameter() = any(TwistedResourceRequestHandler handler).getRequestParameter()
+    }
+
+    override string getSourceType() { result = "twisted.web.server.Request" }
+  }
+
+  /**
+   * Taint propagation for `twisted.web.server.Request`.
+   */
+  private class TwistedRequestAdditionalTaintStep extends TaintTracking::AdditionalTaintStep {
+    override predicate step(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
+      // Methods
+      //
+      // TODO: When we have tools that make it easy, model these properly to handle
+      // `meth = obj.meth; meth()`. Until then, we'll use this more syntactic approach
+      // (since it allows us to at least capture the most common cases).
+      nodeFrom = Request::instance() and
+      exists(DataFlow::AttrRead attr | attr.getObject() = nodeFrom |
+        // normal (non-async) methods
+        attr.getAttributeName() in [
+            "getCookie", "getHeader", "getAllHeaders", "getUser", "getPassword", "getHost",
+            "getRequestHostname"
+          ] and
+        nodeTo.(DataFlow::CallCfgNode).getFunction() = attr
+      )
+      or
+      // Attributes
+      nodeFrom = Request::instance() and
+      nodeTo.(DataFlow::AttrRead).getObject() = nodeFrom and
+      nodeTo.(DataFlow::AttrRead).getAttributeName() in [
+          "uri", "path", "prepath", "postpath", "content", "args", "received_cookies",
+          "requestHeaders", "user", "password", "host"
+        ]
+    }
+  }
+
+  /**
+   * A parameter of a request handler method (on a `twisted.web.resource.Resource` subclass)
+   * that is also given remote user input. (a bit like RoutedParameter).
+   */
+  class TwistedResourceRequestHandlerExtraSources extends RemoteFlowSource::Range,
+    DataFlow::ParameterNode {
+    TwistedResourceRequestHandlerExtraSources() {
+      exists(TwistedResourceRequestHandler func, int i |
+        func.getName() in ["getChild", "getChildWithDefault"] and i = 1
+        or
+        func.getName() = "getDynamicEntity" and i = 1
+      |
+        this.getParameter() = func.getArg(i)
+      )
+    }
+
+    override string getSourceType() { result = "twisted Resource method extra parameter" }
+  }
+
+  // ---------------------------------------------------------------------------
+  // response modeling
+  // ---------------------------------------------------------------------------
+  /**
+   * Implicit response from returns of render methods.
+   */
+  private class TwistedResourceRenderMethodReturn extends HTTP::Server::HttpResponse::Range,
+    DataFlow::CfgNode {
+    TwistedResourceRenderMethodReturn() {
+      this.asCfgNode() = any(TwistedResourceRenderMethod meth).getAReturnValueFlowNode()
+    }
+
+    override DataFlow::Node getBody() { result = this }
+
+    override DataFlow::Node getMimetypeOrContentTypeArg() { none() }
+
+    override string getMimetypeDefault() { result = "text/html" }
+  }
+
+  /**
+   * A call to the `twisted.web.server.Request.write` function.
+   *
+   * See https://twistedmatrix.com/documents/21.2.0/api/twisted.web.server.Request.html#write
+   */
+  class TwistedRequestWriteCall extends HTTP::Server::HttpResponse::Range, DataFlow::CallCfgNode {
+    TwistedRequestWriteCall() {
+      // TODO: When we have tools that make it easy, model these properly to handle
+      // `meth = obj.meth; meth()`. Until then, we'll use this more syntactic approach
+      // (since it allows us to at least capture the most common cases).
+      exists(DataFlow::AttrRead read |
+        this.getFunction() = read and
+        read.getObject() = Request::instance() and
+        read.getAttributeName() = "write"
+      )
+    }
+
+    override DataFlow::Node getBody() {
+      result.asCfgNode() in [node.getArg(0), node.getArgByName("data")]
+    }
+
+    override DataFlow::Node getMimetypeOrContentTypeArg() { none() }
+
+    override string getMimetypeDefault() { result = "text/html" }
+  }
+
+  /**
+   * A call to the `redirect` function on a twisted request.
+   *
+   * See https://twistedmatrix.com/documents/21.2.0/api/twisted.web.http.Request.html#redirect
+   */
+  class TwistedRequestRedirectCall extends HTTP::Server::HttpRedirectResponse::Range,
+    DataFlow::CallCfgNode {
+    TwistedRequestRedirectCall() {
+      // TODO: When we have tools that make it easy, model these properly to handle
+      // `meth = obj.meth; meth()`. Until then, we'll use this more syntactic approach
+      // (since it allows us to at least capture the most common cases).
+      exists(DataFlow::AttrRead read |
+        this.getFunction() = read and
+        read.getObject() = Request::instance() and
+        read.getAttributeName() = "redirect"
+      )
+    }
+
+    override DataFlow::Node getBody() { none() }
+
+    override DataFlow::Node getRedirectLocation() {
+      result.asCfgNode() in [node.getArg(0), node.getArgByName("url")]
+    }
+
+    override DataFlow::Node getMimetypeOrContentTypeArg() { none() }
+
+    override string getMimetypeDefault() { result = "text/html" }
+  }
+}
diff --git a/python/ql/test/library-tests/frameworks/twisted/ConceptsTest.expected b/python/ql/test/library-tests/frameworks/twisted/ConceptsTest.expected
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/python/ql/test/library-tests/frameworks/twisted/ConceptsTest.ql b/python/ql/test/library-tests/frameworks/twisted/ConceptsTest.ql
new file mode 100644
index 000000000000..1e2c1fab3eeb
--- /dev/null
+++ b/python/ql/test/library-tests/frameworks/twisted/ConceptsTest.ql
@@ -0,0 +1,12 @@
+import python
+import experimental.meta.ConceptsTest
+
+class DedicatedResponseTest extends HttpServerHttpResponseTest {
+  DedicatedResponseTest() { file.getShortName() = "response_test.py" }
+}
+
+class OtherResponseTest extends HttpServerHttpResponseTest {
+  OtherResponseTest() { not this instanceof DedicatedResponseTest }
+
+  override string getARelevantTag() { result = "HttpResponse" }
+}
diff --git a/python/ql/test/library-tests/frameworks/twisted/InlineTaintTest.expected b/python/ql/test/library-tests/frameworks/twisted/InlineTaintTest.expected
new file mode 100644
index 000000000000..79d760d87f42
--- /dev/null
+++ b/python/ql/test/library-tests/frameworks/twisted/InlineTaintTest.expected
@@ -0,0 +1,3 @@
+argumentToEnsureNotTaintedNotMarkedAsSpurious
+untaintedArgumentToEnsureTaintedNotMarkedAsMissing
+failures
diff --git a/python/ql/test/library-tests/frameworks/twisted/InlineTaintTest.ql b/python/ql/test/library-tests/frameworks/twisted/InlineTaintTest.ql
new file mode 100644
index 000000000000..027ad8667be6
--- /dev/null
+++ b/python/ql/test/library-tests/frameworks/twisted/InlineTaintTest.ql
@@ -0,0 +1 @@
+import experimental.meta.InlineTaintTest
diff --git a/python/ql/test/library-tests/frameworks/twisted/response_test.py b/python/ql/test/library-tests/frameworks/twisted/response_test.py
new file mode 100644
index 000000000000..f845f7f693a2
--- /dev/null
+++ b/python/ql/test/library-tests/frameworks/twisted/response_test.py
@@ -0,0 +1,75 @@
+from twisted.web.server import Site, Request, NOT_DONE_YET
+from twisted.web.resource import Resource
+from twisted.internet import reactor, endpoints, defer
+
+
+root = Resource()
+
+class Now(Resource):
+    def render(self, request: Request): # $ requestHandler
+        return b"now" # $ HttpResponse mimetype=text/html responseBody=b"now"
+
+
+class AlsoNow(Resource):
+    def render(self, request: Request): # $ requestHandler
+        request.write(b"also now") # $ HttpResponse mimetype=text/html responseBody=b"also now"
+        return b"" # $ HttpResponse mimetype=text/html responseBody=b""
+
+
+def process_later(request: Request):
+    print("process_later called")
+    request.write(b"later") # $ MISSING: responseBody=b"later"
+    request.finish()
+
+
+class Later(Resource):
+    def render(self, request: Request): # $ requestHandler
+        # process the request in 1 second
+        print("setting up callback for process_later")
+        reactor.callLater(1, process_later, request)
+        return NOT_DONE_YET # $ SPURIOUS: HttpResponse mimetype=text/html responseBody=NOT_DONE_YET
+
+
+class PlainText(Resource):
+    def render(self, request: Request): # $ requestHandler
+        request.setHeader(b"content-type", "text/plain")
+        return b"this is plain text" # $ HttpResponse responseBody=b"this is plain text" SPURIOUS: mimetype=text/html MISSING: mimetype=text/plain
+
+
+class Redirect(Resource):
+    def render_GET(self, request: Request): # $ requestHandler
+        request.redirect("/new-location") # $ HttpRedirectResponse redirectLocation="/new-location" HttpResponse mimetype=text/html
+        # By default, this `hello` output is not returned... not even when
+        # requested with curl.
+        return b"hello" # $ SPURIOUS: HttpResponse mimetype=text/html responseBody=b"hello"
+
+
+class NonHttpBodyOutput(Resource):
+    """Examples of providing values in response that is not in the body
+    """
+    def render_GET(self, request: Request): # $ requestHandler
+        request.responseHeaders.addRawHeader("key", "value")
+        request.setHeader("key2", "value")
+
+        request.addCookie("key", "value")
+        request.cookies.append(b"key2=value")
+
+        return b"" # $ HttpResponse mimetype=text/html responseBody=b""
+
+
+root.putChild(b"now", Now())
+root.putChild(b"also-now", AlsoNow())
+root.putChild(b"later", Later())
+root.putChild(b"plain-text", PlainText())
+root.putChild(b"redirect", Redirect())
+root.putChild(b"non-body", NonHttpBodyOutput())
+
+
+if __name__ == "__main__":
+    factory = Site(root)
+    endpoint = endpoints.TCP4ServerEndpoint(reactor, 8880)
+    endpoint.listen(factory)
+
+    print("Will run on http://localhost:8880")
+
+    reactor.run()
diff --git a/python/ql/test/library-tests/frameworks/twisted/routing_test.py b/python/ql/test/library-tests/frameworks/twisted/routing_test.py
new file mode 100644
index 000000000000..bcbcc6521afc
--- /dev/null
+++ b/python/ql/test/library-tests/frameworks/twisted/routing_test.py
@@ -0,0 +1,47 @@
+from twisted.web.server import Site, Request
+from twisted.web.resource import Resource
+from twisted.internet import reactor, endpoints
+
+
+root = Resource()
+
+
+class Foo(Resource):
+    def render(self, request: Request): # $ requestHandler
+        print(f"{request.content=}")
+        print(f"{request.cookies=}")
+        print(f"{request.received_cookies=}")
+        return b"I am Foo" # $ HttpResponse
+
+
+root.putChild(b"foo", Foo())
+
+
+class Child(Resource):
+    def __init__(self, name):
+        self.name = name.decode("utf-8")
+
+    def render_GET(self, request): # $ requestHandler
+        return f"Hi, I'm child '{self.name}'".encode("utf-8") # $ HttpResponse
+
+
+class Parent(Resource):
+    def getChild(self, path, request): # $ requestHandler
+        print(path, type(path))
+        return Child(path)
+
+    def render_GET(self, request): # $ requestHandler
+        return b"Hi, I'm parent" # $ HttpResponse
+
+
+root.putChild(b"parent", Parent())
+
+
+if __name__ == "__main__":
+    factory = Site(root)
+    endpoint = endpoints.TCP4ServerEndpoint(reactor, 8880)
+    endpoint.listen(factory)
+
+    print("Will run on http://localhost:8880")
+
+    reactor.run()
diff --git a/python/ql/test/library-tests/frameworks/twisted/taint_test.py b/python/ql/test/library-tests/frameworks/twisted/taint_test.py
new file mode 100644
index 000000000000..cdc6592f90e4
--- /dev/null
+++ b/python/ql/test/library-tests/frameworks/twisted/taint_test.py
@@ -0,0 +1,70 @@
+from twisted.web.resource import Resource
+from twisted.web.server import Request
+
+class MyTaintTest(Resource):
+    def getChild(self, path, request): # $ requestHandler
+        ensure_tainted(path, request) # $ tainted
+
+    def render(self, request): # $ requestHandler
+        ensure_tainted(request) # $ tainted
+
+    def render_GET(self, request: Request): # $ requestHandler
+        # see https://twistedmatrix.com/documents/21.2.0/api/twisted.web.server.Request.html
+        ensure_tainted(
+            request, # $ tainted
+
+            request.uri, # $ tainted
+            request.path, # $ tainted
+            request.prepath, # $ tainted
+            request.postpath, # $ tainted
+
+            # file-like
+            request.content, # $ tainted
+            request.content.read(), # $ MISSING: tainted
+
+            # Dict[bytes, List[bytes]] (for query args)
+            request.args, # $ tainted
+            request.args[b"key"], # $ tainted
+            request.args[b"key"][0], # $ tainted
+            request.args.get(b"key"), # $ tainted
+            request.args.get(b"key")[0], # $ tainted
+
+            request.received_cookies, # $ tainted
+            request.received_cookies["key"], # $ tainted
+            request.received_cookies.get("key"), # $ tainted
+            request.getCookie(b"key"), # $ tainted
+
+            # twisted.web.http_headers.Headers
+            # see https://twistedmatrix.com/documents/21.2.0/api/twisted.web.http_headers.Headers.html
+            request.requestHeaders, # $ tainted
+            request.requestHeaders.getRawHeaders("key"), # $ MISSING: tainted
+            request.requestHeaders.getRawHeaders("key")[0], # $ MISSING: tainted
+            request.requestHeaders.getAllRawHeaders(), # $ MISSING: tainted
+            list(request.requestHeaders.getAllRawHeaders()), # $ MISSING: tainted
+
+            request.getHeader("key"), # $ tainted
+            request.getAllHeaders(), # $ tainted
+            request.getAllHeaders()["key"], # $ tainted
+
+            request.user, # $ tainted
+            request.getUser(), # $ tainted
+
+            request.password, # $ tainted
+            request.getPassword(), # $ tainted
+
+            request.host, # $ tainted
+            request.getHost(), # $ tainted
+            request.getRequestHostname(), # $ tainted
+        )
+
+        # technically user-controlled, but unlikely to lead to vulnerabilities.
+        ensure_not_tainted(
+            request.method,
+        )
+
+        # not tainted at all
+        ensure_not_tainted(
+            # outgoing things
+            request.cookies,
+            request.responseHeaders,
+        )