diff --git a/cmd/protoc-gen-go/main.go b/cmd/protoc-gen-go/main.go
index d91e7a1..c243e00 100644
--- a/cmd/protoc-gen-go/main.go
+++ b/cmd/protoc-gen-go/main.go
@@ -11,6 +11,7 @@
 	"compress/gzip"
 	"crypto/sha256"
 	"encoding/hex"
+	"flag"
 	"fmt"
 	"strconv"
 	"strings"
@@ -24,7 +25,13 @@
 const protoPackage = "github.com/golang/protobuf/proto"
 
 func main() {
-	protogen.Run(func(gen *protogen.Plugin) error {
+	var flags flag.FlagSet
+	// TODO: Decide what to do for backwards compatibility with plugins=grpc.
+	flags.String("plugins", "", "")
+	opts := &protogen.Options{
+		ParamFunc: flags.Set,
+	}
+	protogen.Run(opts, func(gen *protogen.Plugin) error {
 		for _, f := range gen.Files {
 			if !f.Generate {
 				continue
diff --git a/protogen/protogen.go b/protogen/protogen.go
index ada17ba..ee4c735 100644
--- a/protogen/protogen.go
+++ b/protogen/protogen.go
@@ -41,14 +41,16 @@
 //
 // If a failure occurs while reading or writing, Run prints an error to
 // os.Stderr and calls os.Exit(1).
-func Run(f func(*Plugin) error) {
-	if err := run(f); err != nil {
+//
+// Passing a nil options is equivalent to passing a zero-valued one.
+func Run(opts *Options, f func(*Plugin) error) {
+	if err := run(opts, f); err != nil {
 		fmt.Fprintf(os.Stderr, "%s: %v\n", filepath.Base(os.Args[0]), err)
 		os.Exit(1)
 	}
 }
 
-func run(f func(*Plugin) error) error {
+func run(opts *Options, f func(*Plugin) error) error {
 	in, err := ioutil.ReadAll(os.Stdin)
 	if err != nil {
 		return err
@@ -57,7 +59,7 @@
 	if err := proto.Unmarshal(in, req); err != nil {
 		return err
 	}
-	gen, err := New(req)
+	gen, err := New(req, opts)
 	if err != nil {
 		return err
 	}
@@ -98,15 +100,47 @@
 	err      error
 }
 
+// Options are optional parameters to New.
+type Options struct {
+	// If ParamFunc is non-nil, it will be called with each unknown
+	// generator parameter.
+	//
+	// Plugins for protoc can accept parameters from the command line,
+	// passed in the --<lang>_out protoc, separated from the output
+	// directory with a colon; e.g.,
+	//
+	//   --go_out=<param1>=<value1>,<param2>=<value2>:<output_directory>
+	//
+	// Parameters passed in this fashion as a comma-separated list of
+	// key=value pairs will be passed to the ParamFunc.
+	//
+	// The (flag.FlagSet).Set method matches this function signature,
+	// so parameters can be converted into flags as in the following:
+	//
+	//   var flags flag.FlagSet
+	//   value := flags.Bool("param", false, "")
+	//   opts := &protogen.Options{
+	//     ParamFunc: flags.Set,
+	//   }
+	//   protogen.Run(opts, func(p *protogen.Plugin) error {
+	//     if *value { ... }
+	//   })
+	ParamFunc func(name, value string) error
+}
+
 // New returns a new Plugin.
-func New(req *pluginpb.CodeGeneratorRequest) (*Plugin, error) {
+//
+// Passing a nil Options is equivalent to passing a zero-valued one.
+func New(req *pluginpb.CodeGeneratorRequest, opts *Options) (*Plugin, error) {
+	if opts == nil {
+		opts = &Options{}
+	}
 	gen := &Plugin{
 		Request:     req,
 		filesByName: make(map[string]*File),
 		fileReg:     protoregistry.NewFiles(),
 	}
 
-	// TODO: Figure out how to pass parameters to the generator.
 	packageNames := make(map[string]GoPackageName) // filename -> package name
 	importPaths := make(map[string]GoImportPath)   // filename -> import path
 	var packageImportPath GoImportPath
@@ -132,15 +166,18 @@
 			default:
 				return nil, fmt.Errorf(`unknown path type %q: want "import" or "source_relative"`, value)
 			}
-		case "plugins":
-			// TODO
 		case "annotate_code":
 			// TODO
 		default:
-			if param[0] != 'M' {
-				return nil, fmt.Errorf("unknown parameter %q", param)
+			if param[0] == 'M' {
+				importPaths[param[1:]] = GoImportPath(value)
+				continue
 			}
-			importPaths[param[1:]] = GoImportPath(value)
+			if opts.ParamFunc != nil {
+				if err := opts.ParamFunc(param, value); err != nil {
+					return nil, err
+				}
+			}
 		}
 	}
 
diff --git a/protogen/protogen_test.go b/protogen/protogen_test.go
index 05d0bf2..f35068a 100644
--- a/protogen/protogen_test.go
+++ b/protogen/protogen_test.go
@@ -5,6 +5,7 @@
 package protogen
 
 import (
+	"flag"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -18,8 +19,45 @@
 	pluginpb "github.com/golang/protobuf/protoc-gen-go/plugin"
 )
 
+func TestPluginParameters(t *testing.T) {
+	var flags flag.FlagSet
+	value := flags.Int("integer", 0, "")
+	opts := &Options{
+		ParamFunc: flags.Set,
+	}
+	const params = "integer=2"
+	_, err := New(&pluginpb.CodeGeneratorRequest{
+		Parameter: proto.String(params),
+	}, opts)
+	if err != nil {
+		t.Errorf("New(generator parameters %q): %v", params, err)
+	}
+	if *value != 2 {
+		t.Errorf("New(generator parameters %q): integer=%v, want 2", params, *value)
+	}
+}
+
+func TestPluginParameterErrors(t *testing.T) {
+	for _, parameter := range []string{
+		"unknown=1",
+		"boolean=error",
+	} {
+		var flags flag.FlagSet
+		flags.Bool("boolean", false, "")
+		opts := &Options{
+			ParamFunc: flags.Set,
+		}
+		_, err := New(&pluginpb.CodeGeneratorRequest{
+			Parameter: proto.String(parameter),
+		}, opts)
+		if err == nil {
+			t.Errorf("New(generator parameters %q): want error, got nil", parameter)
+		}
+	}
+}
+
 func TestFiles(t *testing.T) {
-	gen, err := New(makeRequest(t, "testdata/go_package/no_go_package_import.proto"))
+	gen, err := New(makeRequest(t, "testdata/go_package/no_go_package_import.proto"), nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -144,7 +182,7 @@
 		if test.generate {
 			req.FileToGenerate = []string{filename}
 		}
-		gen, err := New(req)
+		gen, err := New(req, nil)
 		if err != nil {
 			t.Errorf("%vNew(req) = %v", context, err)
 			continue
@@ -182,7 +220,7 @@
 			},
 		},
 		FileToGenerate: []string{"dir/file1.proto", "dir/file2.proto"},
-	})
+	}, nil)
 	if err != nil {
 		t.Fatalf("New(req) = %v", err)
 	}
@@ -212,14 +250,14 @@
 			},
 		},
 		FileToGenerate: []string{"dir/file1.proto", "dir/file2.proto"},
-	})
+	}, nil)
 	if err == nil {
 		t.Fatalf("inconsistent package names for the same import path: New(req) = nil, want error")
 	}
 }
 
 func TestImports(t *testing.T) {
-	gen, err := New(&pluginpb.CodeGeneratorRequest{})
+	gen, err := New(&pluginpb.CodeGeneratorRequest{}, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -309,7 +347,7 @@
 
 func init() {
 	if os.Getenv("RUN_AS_PROTOC_PLUGIN") != "" {
-		Run(func(p *Plugin) error {
+		Run(nil, func(p *Plugin) error {
 			g := p.NewGeneratedFile("request", "")
 			return proto.MarshalText(g, p.Request)
 		})