diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 3e0cc62a60..05c61bb5ae 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -209,6 +209,7 @@ set(TINT_TEST_SRCS
   ast/else_statement_test.cc
   ast/entry_point_test.cc
   ast/fallthrough_statement_test.cc
+  ast/function_test.cc
   ast/import_test.cc
   ast/int_literal_test.cc
   ast/location_decoration_test.cc
diff --git a/src/ast/function.cc b/src/ast/function.cc
index f53ba0b0bb..4d13d139cc 100644
--- a/src/ast/function.cc
+++ b/src/ast/function.cc
@@ -37,6 +37,15 @@ Function::Function(const Source& source,
 Function::~Function() = default;
 
 bool Function::IsValid() const {
+  for (const auto& param : params_) {
+    if (param == nullptr || !param->IsValid())
+      return false;
+  }
+  for (const auto& stmt : body_) {
+    if (stmt == nullptr || !stmt->IsValid())
+      return false;
+  }
+
   if (name_.length() == 0) {
     return false;
   }
@@ -48,21 +57,28 @@ bool Function::IsValid() const {
 
 void Function::to_str(std::ostream& out, size_t indent) const {
   make_indent(out, indent);
-  out << "function -> " << return_type_->type_name() << "{" << std::endl;
-  make_indent(out, indent + 2);
-  out << name_ << std::endl;
+  out << "Function " << name_ << " -> " << return_type_->type_name()
+      << std::endl;
 
-  for (const auto& param : params_)
-    param->to_str(out, indent + 2);
+  make_indent(out, indent);
+  out << "(";
 
-  make_indent(out, indent + 2);
+  if (params_.size() > 0) {
+    out << std::endl;
+
+    for (const auto& param : params_)
+      param->to_str(out, indent + 2);
+
+    make_indent(out, indent);
+  }
+  out << ")" << std::endl;
+
+  make_indent(out, indent);
   out << "{" << std::endl;
 
   for (const auto& stmt : body_)
-    stmt->to_str(out, indent + 4);
+    stmt->to_str(out, indent + 2);
 
-  make_indent(out, indent + 2);
-  out << "}" << std::endl;
   make_indent(out, indent);
   out << "}" << std::endl;
 }
diff --git a/src/ast/function_test.cc b/src/ast/function_test.cc
new file mode 100644
index 0000000000..d2bd9aacf7
--- /dev/null
+++ b/src/ast/function_test.cc
@@ -0,0 +1,206 @@
+// Copyright 2020 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "src/ast/function.h"
+
+#include "gtest/gtest.h"
+#include "src/ast/nop_statement.h"
+#include "src/ast/type/i32_type.h"
+#include "src/ast/type/void_type.h"
+#include "src/ast/variable.h"
+
+namespace tint {
+namespace ast {
+
+using FunctionTest = testing::Test;
+
+TEST_F(FunctionTest, Creation) {
+  type::VoidType void_type;
+  type::I32Type i32;
+
+  std::vector<std::unique_ptr<Variable>> params;
+  params.push_back(
+      std::make_unique<Variable>("var", StorageClass::kNone, &i32));
+  auto var_ptr = params[0].get();
+
+  Function f("func", std::move(params), &void_type);
+  EXPECT_EQ(f.name(), "func");
+  ASSERT_EQ(f.params().size(), 1);
+  EXPECT_EQ(f.return_type(), &void_type);
+  EXPECT_EQ(f.params()[0].get(), var_ptr);
+}
+
+TEST_F(FunctionTest, Creation_WithSource) {
+  type::VoidType void_type;
+  type::I32Type i32;
+
+  std::vector<std::unique_ptr<Variable>> params;
+  params.push_back(
+      std::make_unique<Variable>("var", StorageClass::kNone, &i32));
+
+  Function f(Source{20, 2}, "func", std::move(params), &void_type);
+  auto src = f.source();
+  EXPECT_EQ(src.line, 20);
+  EXPECT_EQ(src.column, 2);
+}
+
+TEST_F(FunctionTest, IsValid) {
+  type::VoidType void_type;
+  type::I32Type i32;
+
+  std::vector<std::unique_ptr<Variable>> params;
+  params.push_back(
+      std::make_unique<Variable>("var", StorageClass::kNone, &i32));
+
+  std::vector<std::unique_ptr<Statement>> body;
+  body.push_back(std::make_unique<NopStatement>());
+
+  Function f("func", std::move(params), &void_type);
+  f.set_body(std::move(body));
+  EXPECT_TRUE(f.IsValid());
+}
+
+TEST_F(FunctionTest, IsValid_EmptyName) {
+  type::VoidType void_type;
+  type::I32Type i32;
+
+  std::vector<std::unique_ptr<Variable>> params;
+  params.push_back(
+      std::make_unique<Variable>("var", StorageClass::kNone, &i32));
+
+  Function f("", std::move(params), &void_type);
+  EXPECT_FALSE(f.IsValid());
+}
+
+TEST_F(FunctionTest, IsValid_MissingReturnType) {
+  type::I32Type i32;
+
+  std::vector<std::unique_ptr<Variable>> params;
+  params.push_back(
+      std::make_unique<Variable>("var", StorageClass::kNone, &i32));
+
+  Function f("func", std::move(params), nullptr);
+  EXPECT_FALSE(f.IsValid());
+}
+
+TEST_F(FunctionTest, IsValid_NullParam) {
+  type::VoidType void_type;
+  type::I32Type i32;
+
+  std::vector<std::unique_ptr<Variable>> params;
+  params.push_back(
+      std::make_unique<Variable>("var", StorageClass::kNone, &i32));
+  params.push_back(nullptr);
+
+  Function f("func", std::move(params), &void_type);
+  EXPECT_FALSE(f.IsValid());
+}
+
+TEST_F(FunctionTest, IsValid_InvalidParam) {
+  type::VoidType void_type;
+
+  std::vector<std::unique_ptr<Variable>> params;
+  params.push_back(
+      std::make_unique<Variable>("var", StorageClass::kNone, nullptr));
+
+  Function f("func", std::move(params), &void_type);
+  EXPECT_FALSE(f.IsValid());
+}
+
+TEST_F(FunctionTest, IsValid_NullBodyStatement) {
+  type::VoidType void_type;
+  type::I32Type i32;
+
+  std::vector<std::unique_ptr<Variable>> params;
+  params.push_back(
+      std::make_unique<Variable>("var", StorageClass::kNone, &i32));
+
+  std::vector<std::unique_ptr<Statement>> body;
+  body.push_back(std::make_unique<NopStatement>());
+  body.push_back(nullptr);
+
+  Function f("func", std::move(params), &void_type);
+  f.set_body(std::move(body));
+  EXPECT_FALSE(f.IsValid());
+}
+
+TEST_F(FunctionTest, IsValid_InvalidBodyStatement) {
+  type::VoidType void_type;
+  type::I32Type i32;
+
+  std::vector<std::unique_ptr<Variable>> params;
+  params.push_back(
+      std::make_unique<Variable>("var", StorageClass::kNone, &i32));
+
+  std::vector<std::unique_ptr<Statement>> body;
+  body.push_back(std::make_unique<NopStatement>());
+  body.push_back(nullptr);
+
+  Function f("func", std::move(params), &void_type);
+  f.set_body(std::move(body));
+  EXPECT_FALSE(f.IsValid());
+}
+
+TEST_F(FunctionTest, ToStr) {
+  type::VoidType void_type;
+  type::I32Type i32;
+
+  std::vector<std::unique_ptr<Statement>> body;
+  body.push_back(std::make_unique<NopStatement>());
+
+  Function f("func", {}, &void_type);
+  f.set_body(std::move(body));
+
+  std::ostringstream out;
+  f.to_str(out, 2);
+  EXPECT_EQ(out.str(), R"(  Function func -> __void
+  ()
+  {
+    Nop{}
+  }
+)");
+}
+
+TEST_F(FunctionTest, ToStr_WithParams) {
+  type::VoidType void_type;
+  type::I32Type i32;
+
+  std::vector<std::unique_ptr<Variable>> params;
+  params.push_back(
+      std::make_unique<Variable>("var", StorageClass::kNone, &i32));
+
+  std::vector<std::unique_ptr<Statement>> body;
+  body.push_back(std::make_unique<NopStatement>());
+
+  Function f("func", std::move(params), &void_type);
+  f.set_body(std::move(body));
+
+  std::ostringstream out;
+  f.to_str(out, 2);
+  EXPECT_EQ(out.str(), R"(  Function func -> __void
+  (
+    Variable{
+      var
+      none
+      __i32
+    }
+  )
+  {
+    Nop{}
+  }
+)");
+}
+
+}  // namespace ast
+}  // namespace tint