diff --git a/Project.toml b/Project.toml
index e0727aa64..f5273df1d 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "0.10.1"
+version = "0.10.2"
 
 [deps]
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"
diff --git a/docs/src/api.md b/docs/src/api.md
index 1fffafd8f..fc6ac50ab 100644
--- a/docs/src/api.md
+++ b/docs/src/api.md
@@ -39,3 +39,8 @@ ChainRulesCore.is_inplaceable_destination
 ChainRulesCore.AbstractTangent
 ChainRulesCore.debug_mode
 ```
+
+## Deprecated
+```@docs
+ChainRulesCore.extern
+```
diff --git a/src/deprecated.jl b/src/deprecated.jl
index ea7df0674..8e60d4073 100644
--- a/src/deprecated.jl
+++ b/src/deprecated.jl
@@ -1 +1,53 @@
 Base.@deprecate_binding NO_FIELDS NoTangent()
+
+const EXTERN_DEPRECATION = "`extern` is deprecated, use `unthunk` or `backing` instead, " *
+    "depending on the use case."
+
+"""
+    extern(x)
+
+Makes a best effort attempt to convert a differential into a primal value.
+This is not always a well-defined operation.
+For two reasons:
+ - It may not be possible to determine the primal type for a given differential.
+ For example, `Zero` is a valid differential for any primal.
+ - The primal type might not be a vector space, thus might not be a valid differential type.
+ For example, if the primal type is `DateTime`, it's not a valid differential type as two
+ `DateTime` can not be added (fun fact: `Milisecond` is a differential for `DateTime`).
+
+Where it is defined the operation of `extern` for a primal type `P` should be
+`extern(x) = zero(P) + x`.
+
+!!! note
+    Because of its limitations, `extern` should only really be used for testing.
+    It can be useful, if you know what you are getting out, as it recursively removes
+    thunks, and otherwise makes outputs more consistent with finite differencing.
+
+    The more useful action in general is to call `+`, or in the case of a [`Thunk`](@ref)
+    to call [`unthunk`](@ref).
+
+!!! warning
+    `extern` may return an alias (not necessarily a copy) to data
+    wrapped by `x`, such that mutating `extern(x)` might mutate `x` itself.
+"""
+@inline function extern(x)
+    Base.depwarn(EXTERN_DEPRECATION, :extern)
+    return x
+end
+
+extern(x::ZeroTangent) = (Base.depwarn(EXTERN_DEPRECATION, :extern); return false)  # false is a strong 0. E.g. `false * NaN = 0.0`
+
+function extern(x::NoTangent)
+    Base.depwarn(EXTERN_DEPRECATION, :extern)
+    throw(ArgumentError("Derivative does not exit. Cannot be converted to an external type."))
+end
+
+extern(comp::Tangent) = (Base.depwarn(EXTERN_DEPRECATION, :extern); return backing(map(extern, comp)))  # gives a NamedTuple or Tuple
+
+extern(x::NotImplemented) = (Base.depwarn(EXTERN_DEPRECATION, :extern); throw(NotImplementedException(x)))
+
+@inline extern(x::AbstractThunk) = (Base.depwarn(EXTERN_DEPRECATION, :extern); return extern(unthunk(x)))
+
+
+
+
diff --git a/src/differentials/abstract_differential.jl b/src/differentials/abstract_differential.jl
index 9169ce45e..029d03af9 100644
--- a/src/differentials/abstract_differential.jl
+++ b/src/differentials/abstract_differential.jl
@@ -37,33 +37,4 @@ abstract type AbstractTangent end
 
 Base.:+(x::AbstractTangent) = x
 
-"""
-    extern(x)
-
-Makes a best effort attempt to convert a differential into a primal value.
-This is not always a well-defined operation.
-For two reasons:
- - It may not be possible to determine the primal type for a given differential.
- For example, `Zero` is a valid differential for any primal.
- - The primal type might not be a vector space, thus might not be a valid differential type.
- For example, if the primal type is `DateTime`, it's not a valid differential type as two
- `DateTime` can not be added (fun fact: `Milisecond` is a differential for `DateTime`).
-
-Where it is defined the operation of `extern` for a primal type `P` should be
-`extern(x) = zero(P) + x`.
-
-!!! note
-    Because of its limitations, `extern` should only really be used for testing.
-    It can be useful, if you know what you are getting out, as it recursively removes
-    thunks, and otherwise makes outputs more consistent with finite differencing.
-
-    The more useful action in general is to call `+`, or in the case of a [`Thunk`](@ref)
-    to call [`unthunk`](@ref).
-
-!!! warning
-    `extern` may return an alias (not necessarily a copy) to data
-    wrapped by `x`, such that mutating `extern(x)` might mutate `x` itself.
-"""
-@inline extern(x) = x
-
 @inline Base.conj(x::AbstractTangent) = x
diff --git a/src/differentials/abstract_zero.jl b/src/differentials/abstract_zero.jl
index 46be3b0ad..52ef38adb 100644
--- a/src/differentials/abstract_zero.jl
+++ b/src/differentials/abstract_zero.jl
@@ -35,8 +35,6 @@ A derivative of `ZeroTangent()` does not propagate through the primal function.
 """
 struct ZeroTangent <: AbstractZero end
 
-extern(x::ZeroTangent) = false  # false is a strong 0. E.g. `false * NaN = 0.0`
-
 Base.eltype(::Type{ZeroTangent}) = ZeroTangent
 
 Base.zero(::AbstractTangent) = ZeroTangent()
@@ -72,7 +70,3 @@ arguments.
 ```
 """
 struct NoTangent <: AbstractZero end
-
-function extern(x::NoTangent)
-    throw(ArgumentError("Derivative does not exit. Cannot be converted to an external type."))
-end
diff --git a/src/differentials/composite.jl b/src/differentials/composite.jl
index 9fadf3e76..af1a05d1f 100644
--- a/src/differentials/composite.jl
+++ b/src/differentials/composite.jl
@@ -114,9 +114,6 @@ end
 
 Base.conj(comp::Tangent) = map(conj, comp)
 
-extern(comp::Tangent) = backing(map(extern, comp))  # gives a NamedTuple or Tuple
-
-
 """
     backing(x)
 
diff --git a/src/differentials/notimplemented.jl b/src/differentials/notimplemented.jl
index 6a5b8961f..a2044fbe1 100644
--- a/src/differentials/notimplemented.jl
+++ b/src/differentials/notimplemented.jl
@@ -39,8 +39,6 @@ Base.Broadcast.broadcastable(x::NotImplemented) = Ref(x)
 
 # throw error with debugging information for other standard information
 # (`+`, `-`, `*`, and `dot` are defined in differential_arithmetic.jl)
-extern(x::NotImplemented) = throw(NotImplementedException(x))
-
 Base.:/(x::NotImplemented, ::Any) = throw(NotImplementedException(x))
 Base.:/(::Any, x::NotImplemented) = throw(NotImplementedException(x))
 Base.:/(x::NotImplemented, ::NotImplemented) = throw(NotImplementedException(x))
diff --git a/src/differentials/thunks.jl b/src/differentials/thunks.jl
index 545fb4835..efaebb0c7 100644
--- a/src/differentials/thunks.jl
+++ b/src/differentials/thunks.jl
@@ -30,13 +30,9 @@ end
 
 On `AbstractThunk`s this removes 1 layer of thunking.
 On any other type, it is the identity operation.
-
-In contrast to [`extern`](@ref) this is nonrecursive.
 """
 @inline unthunk(x) = x
 
-@inline extern(x::AbstractThunk) = extern(unthunk(x))
-
 Base.conj(x::AbstractThunk) = @thunk(conj(unthunk(x)))
 Base.adjoint(x::AbstractThunk) = @thunk(adjoint(unthunk(x)))
 Base.transpose(x::AbstractThunk) = @thunk(transpose(unthunk(x)))
@@ -54,16 +50,11 @@ It wraps a zero argument closure that when invoked returns a differential.
 Calling a thunk, calls the wrapped closure.
 If you are unsure if you have a `Thunk`, call [`unthunk`](@ref) which is a no-op when the
 argument is not a `Thunk`.
-If you need to unthunk recursively, call [`extern`](@ref), which also externs the differial
-that the closure returns.
 
 ```jldoctest
 julia> t = @thunk(@thunk(3))
 Thunk(var"#4#6"())
 
-julia> extern(t)
-3
-
 julia> t()
 Thunk(var"#5#7"())
 
diff --git a/test/deprecated.jl b/test/deprecated.jl
index 42555a3c9..7f7eb6c61 100644
--- a/test/deprecated.jl
+++ b/test/deprecated.jl
@@ -1,3 +1,24 @@
 @testset "NO_FIELDS" begin
     @test (@test_deprecated NO_FIELDS) isa NoTangent
 end
+
+@testset "extern" begin
+    @test extern(@thunk(3)) == 3
+    @test extern(@thunk(@thunk(3))) == 3
+
+    @test extern(Tangent{Foo}(x=2.0)) == (;x=2.0)
+    @test extern(Tangent{Tuple{Float64,}}(2.0)) == (2.0,)
+    @test extern(Tangent{Dict}(Dict(4 => 3))) == Dict(4 => 3)
+
+    # with differentials on the inside
+    @test extern(Tangent{Foo}(x=@thunk(0+2.0))) == (;x=2.0)
+    @test extern(Tangent{Tuple{Float64,}}(@thunk(0+2.0))) == (2.0,)
+    @test extern(Tangent{Dict}(Dict(4 => @thunk(3)))) == Dict(4 => 3)
+
+    z = ZeroTangent()
+    @test extern(z) === false
+    dne = NoTangent()
+    @test_throws Exception extern(dne)
+    E = ChainRulesCore.NotImplementedException
+    @test_throws E extern(ni)
+end
diff --git a/test/differentials/abstract_zero.jl b/test/differentials/abstract_zero.jl
index 7a1adb577..9eb037651 100644
--- a/test/differentials/abstract_zero.jl
+++ b/test/differentials/abstract_zero.jl
@@ -6,7 +6,6 @@
 
     @testset "ZeroTangent" begin
         z = ZeroTangent()
-        @test extern(z) === false
         @test z + z === z
         @test z + 1 === 1
         @test 1 + z === 1
@@ -64,7 +63,6 @@
 
     @testset "NoTangent" begin
         dne = NoTangent()
-        @test_throws Exception extern(dne)
         @test dne + dne == dne
         @test dne + 1 == 1
         @test 1 + dne == 1
diff --git a/test/differentials/composite.jl b/test/differentials/composite.jl
index 9ae2b15c5..37da31d47 100644
--- a/test/differentials/composite.jl
+++ b/test/differentials/composite.jl
@@ -123,17 +123,6 @@ end
         )
     end
 
-    @testset "extern" begin
-        @test extern(Tangent{Foo}(x=2.0)) == (;x=2.0)
-        @test extern(Tangent{Tuple{Float64,}}(2.0)) == (2.0,)
-        @test extern(Tangent{Dict}(Dict(4 => 3))) == Dict(4 => 3)
-
-        # with differentials on the inside
-        @test extern(Tangent{Foo}(x=@thunk(0+2.0))) == (;x=2.0)
-        @test extern(Tangent{Tuple{Float64,}}(@thunk(0+2.0))) == (2.0,)
-        @test extern(Tangent{Dict}(Dict(4 => @thunk(3)))) == Dict(4 => 3)
-    end
-
     @testset "canonicalize" begin
         # Testing iterate via collect
         @test ==(
diff --git a/test/differentials/notimplemented.jl b/test/differentials/notimplemented.jl
index 7ed33350a..fe3668518 100644
--- a/test/differentials/notimplemented.jl
+++ b/test/differentials/notimplemented.jl
@@ -49,7 +49,6 @@
 
         # unsupported operations
         E = ChainRulesCore.NotImplementedException
-        @test_throws E extern(ni)
         @test_throws E +ni
         @test_throws E -ni
         @test_throws E ni - rand()
diff --git a/test/differentials/thunks.jl b/test/differentials/thunks.jl
index 1bc79290d..2fb532fb0 100644
--- a/test/differentials/thunks.jl
+++ b/test/differentials/thunks.jl
@@ -6,11 +6,6 @@
         @test occursin(r"Thunk\(.*rand.*\)", rep)
     end
 
-    @testset "Externing" begin
-        @test extern(@thunk(3)) == 3
-        @test extern(@thunk(@thunk(3))) == 3
-    end
-
     @testset "unthunk" begin
         @test unthunk(@thunk(3)) == 3
         @test unthunk(@thunk(@thunk(3))) isa Thunk
@@ -25,7 +20,7 @@
         expected_line = (@__LINE__) + 2  # for testing it is at right palce
         try
             x = @thunk(error())
-            extern(x)
+            unthunk(x)
         catch err
             err isa ErrorException || rethrow()
             st = stacktrace(catch_backtrace())