diff --git a/.gitmodules b/.gitmodules
index ab43f5a5..e53575f1 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -4,3 +4,6 @@
 [submodule "lib/yaml-cpp"]
 	path = lib/yaml-cpp
 	url = https://github.com/jbeder/yaml-cpp
+[submodule "lib/googletest"]
+	path = lib/googletest
+	url = https://github.com/google/googletest
diff --git a/lib/googletest b/lib/googletest
new file mode 160000
index 00000000..42140509
--- /dev/null
+++ b/lib/googletest
@@ -0,0 +1 @@
+Subproject commit 42140509b6fc2337f8cffe913f6ce1dbf489513b
diff --git a/test/Makefile b/test/Makefile
new file mode 100644
index 00000000..efb58a36
--- /dev/null
+++ b/test/Makefile
@@ -0,0 +1,67 @@
+LIBDIR=../lib
+GTEST_MODULE=$(LIBDIR)/googletest
+GTEST_DIR=$(LIBDIR)/googletest/googletest
+GTEST_SOURCEDIR=$(GTEST_DIR)/src
+GTEST_HEADERS=$(GTEST_DIR)/include/gtest/*.h \
+              $(GTEST_DIR)/include/gtest/internal/*.h
+GTEST_SRCS_=$(GTEST_SOURCEDIR)/*.cc $(GTEST_SOURCEDIR)/*.h $(GTEST_HEADERS)
+
+# TODO: Factor out the common variables.
+CPP_NETLIB_DIR=$(LIBDIR)/cpp-netlib
+YAML_CPP_DIR=$(LIBDIR)/yaml-cpp
+
+CPPFLAGS+=-isystem $(GTEST_DIR)/include
+CXXFLAGS=-std=c++11 -g -pthread
+LDLIBS=-lpthread
+
+# Where to find code under test.
+SRC_DIR=../src
+
+TEST_DIR=.
+TEST_SOURCES=$(wildcard $(TEST_DIR)/*_unittest.cc)
+TEST_OBJS=$(TEST_SOURCES:$(TEST_DIR)/%.cc=%.o)
+TESTS=\
+      format_unittest
+
+GTEST_LIB=gtest_lib.a
+
+all: $(TESTS)
+
+# TODO: Running the test prints "Running main() from gtest_main.cc".
+# Figure out how to fix this.
+test: $(TESTS)
+	@for t in $(TESTS); do \
+		echo "Running $$t"; \
+		./$$t; \
+	done
+
+clean:
+	$(RM) $(TESTS) $(GTEST_LIB) $(TEST_OBJS)
+
+purge: clean
+	$(RM) init-submodules
+	git submodule deinit -f $(GTEST_MODULE)
+
+init-submodules:
+	git submodule update --init $(GTEST_MODULE)
+	touch init-submodules
+
+
+$(SRC_DIR)/%.o: $(SRC_DIR)/%.cc
+	cd $(SRC_DIR) && $(MAKE) $(@:$(SRC_DIR)/%=%)
+
+$(GTEST_SOURCEDIR)/gtest-all.cc $(GTEST_SOURCEDIR)/gtest_main.cc: init-submodules
+
+gtest-all.o: $(GTEST_SOURCEDIR)/gtest-all.cc
+	$(CXX) -c $(CPPFLAGS) -I$(GTEST_DIR) $(CXXFLAGS) -o $@ $^
+
+gtest_main.o: $(GTEST_SOURCEDIR)/gtest_main.cc
+	$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c -o $@ $^
+
+$(GTEST_LIB): gtest-all.o gtest_main.o
+	$(AR) $(ARFLAGS) $@ $^
+
+format_unittest: $(GTEST_LIB) format_unittest.o $(SRC_DIR)/format.o
+	$(CXX) $(LDFLAGS) $^ $(LDLIBS) -o $@
+
+.PHONY: all test clean purge
diff --git a/test/format_unittest.cc b/test/format_unittest.cc
new file mode 100644
index 00000000..d2657422
--- /dev/null
+++ b/test/format_unittest.cc
@@ -0,0 +1,38 @@
+#include "../src/format.h"
+#include "gtest/gtest.h"
+
+namespace {
+
+TEST(SubstituteTest, SingleSubstitution) {
+  const std::string format_str("prefix|{{subst}}|suffix");
+  EXPECT_EQ(
+      "prefix|value|suffix",
+      format::Substitute(format_str, {{"subst", "value"}})
+  );
+}
+
+TEST(SubstituteTest, MultipleSubstitutions) {
+  const std::string format_str("prefix|{{first}}|middle|{{second}}|suffix");
+  EXPECT_EQ(
+      "prefix|one|middle|two|suffix",
+      format::Substitute(
+          format_str, {{"first", "one"}, {"second", "two"}})
+  );
+}
+
+TEST(SubstituteTest, UnterminatedPlaceholder) {
+  const std::string format_str("prefix|{{subst|suffix");
+  EXPECT_THROW(
+      format::Substitute(format_str, {{"subst", "value"}}),
+      format::Exception
+  );
+}
+
+TEST(SubstituteTest, UnknownParameter) {
+  const std::string format_str("prefix|{{subst}}|suffix");
+  EXPECT_THROW(
+      format::Substitute(format_str, {{"unknown", "value"}}),
+      format::Exception
+  );
+}
+} // namespace