diff --git a/internal/lsp/diagnostics.go b/internal/lsp/diagnostics.go
index a1a0e9d2..06f39b73 100644
--- a/internal/lsp/diagnostics.go
+++ b/internal/lsp/diagnostics.go
@@ -5,6 +5,7 @@
 package lsp
 
 import (
+	"fmt"
 	"go/token"
 	"strconv"
 	"strings"
@@ -19,6 +20,9 @@ func diagnostics(v *source.View, uri source.URI) (map[string][]protocol.Diagnost
 	if err != nil {
 		return nil, err
 	}
+	if pkg == nil {
+		return nil, fmt.Errorf("package for %v not found", uri)
+	}
 	reports := make(map[string][]protocol.Diagnostic)
 	for _, filename := range pkg.GoFiles {
 		reports[filename] = []protocol.Diagnostic{}
diff --git a/internal/lsp/server.go b/internal/lsp/server.go
index c07748ec..9ee806ae 100644
--- a/internal/lsp/server.go
+++ b/internal/lsp/server.go
@@ -51,6 +51,7 @@ func (s *server) Initialize(ctx context.Context, params *protocol.InitializePara
 			CompletionProvider: protocol.CompletionOptions{
 				TriggerCharacters: []string{"."},
 			},
+			DefinitionProvider: true,
 		},
 	}, nil
 }
@@ -170,8 +171,18 @@ func (s *server) SignatureHelp(context.Context, *protocol.TextDocumentPositionPa
 	return nil, notImplemented("SignatureHelp")
 }
 
-func (s *server) Definition(context.Context, *protocol.TextDocumentPositionParams) ([]protocol.Location, error) {
-	return nil, notImplemented("Definition")
+func (s *server) Definition(ctx context.Context, params *protocol.TextDocumentPositionParams) ([]protocol.Location, error) {
+	f := s.view.GetFile(source.URI(params.TextDocument.URI))
+	tok, err := f.GetToken()
+	if err != nil {
+		return nil, err
+	}
+	pos := fromProtocolPosition(tok, params.Position)
+	r, err := source.Definition(ctx, f, pos)
+	if err != nil {
+		return nil, err
+	}
+	return []protocol.Location{toProtocolLocation(s.view, r)}, nil
 }
 
 func (s *server) TypeDefinition(context.Context, *protocol.TextDocumentPositionParams) ([]protocol.Location, error) {
diff --git a/internal/lsp/source/definition.go b/internal/lsp/source/definition.go
new file mode 100644
index 00000000..9886f3d4
--- /dev/null
+++ b/internal/lsp/source/definition.go
@@ -0,0 +1,66 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package source
+
+import (
+	"context"
+	"fmt"
+	"go/ast"
+	"go/token"
+
+	"golang.org/x/tools/go/ast/astutil"
+)
+
+func Definition(ctx context.Context, f *File, pos token.Pos) (Range, error) {
+	fAST, err := f.GetAST()
+	if err != nil {
+		return Range{}, err
+	}
+	pkg, err := f.GetPackage()
+	if err != nil {
+		return Range{}, err
+	}
+	ident, err := findIdentifier(fAST, pos)
+	if err != nil {
+		return Range{}, err
+	}
+	if ident == nil {
+		return Range{}, fmt.Errorf("definition was not a valid identifier")
+	}
+	obj := pkg.TypesInfo.ObjectOf(ident)
+	if obj == nil {
+		return Range{}, fmt.Errorf("no object")
+	}
+	return Range{
+		Start: obj.Pos(),
+		End:   obj.Pos() + token.Pos(len([]byte(obj.Name()))), // TODO: use real range of obj
+	}, nil
+}
+
+// findIdentifier returns the ast.Ident for a position
+// in a file, accounting for a potentially incomplete selector.
+func findIdentifier(f *ast.File, pos token.Pos) (*ast.Ident, error) {
+	path, _ := astutil.PathEnclosingInterval(f, pos, pos)
+	if path == nil {
+		return nil, fmt.Errorf("can't find node enclosing position")
+	}
+	// If the position is not an identifier but immediately follows
+	// an identifier or selector period (as is common when
+	// requesting a completion), use the path to the preceding node.
+	if ident, ok := path[0].(*ast.Ident); ok {
+		return ident, nil
+	}
+	path, _ = astutil.PathEnclosingInterval(f, pos-1, pos-1)
+	if path == nil {
+		return nil, nil
+	}
+	switch prev := path[0].(type) {
+	case *ast.Ident:
+		return prev, nil
+	case *ast.SelectorExpr:
+		return prev.Sel, nil
+	}
+	return nil, nil
+}