diff --git a/cmd/common.go b/cmd/common.go
index 5ba67b9..a96d3c4 100644
--- a/cmd/common.go
+++ b/cmd/common.go
@@ -309,17 +309,30 @@ func processLog(logInfo *certspotter.LogInfo, processCallback certspotter.Proces
 func Main(statePath string, processCallback certspotter.ProcessCallback) int {
 	var err error
 
-	state, err = OpenState(statePath)
+	logs, err := loadLogList()
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "%s: %s\n", os.Args[0], err)
 		return 1
 	}
 
-	logs, err := loadLogList()
+	state, err = OpenState(statePath)
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "%s: %s\n", os.Args[0], err)
 		return 1
 	}
+	locked, err := state.Lock()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "%s: Error locking state directory: %s\n", os.Args[0], err)
+		return 1
+	}
+	if !locked {
+		var otherPidInfo string
+		if otherPid := state.LockingPid(); otherPid != 0 {
+			otherPidInfo = fmt.Sprintf(" (as process ID %d)", otherPid)
+		}
+		fmt.Fprintf(os.Stderr, "%s: Another instance of %s is already running%s; remove the file %s if this is not the case\n", os.Args[0], os.Args[0], otherPidInfo, state.LockFilename())
+		return 1
+	}
 
 	exitCode := 0
 	for i := range logs {
@@ -333,5 +346,10 @@ func Main(statePath string, processCallback certspotter.ProcessCallback) int {
 		}
 	}
 
+	if err := state.Unlock(); err != nil {
+		fmt.Fprintf(os.Stderr, "%s: Error unlocking state directory: %s\n", os.Args[0], err)
+		exitCode |= 1
+	}
+
 	return exitCode
 }
diff --git a/cmd/state.go b/cmd/state.go
index 36bfd36..6ae4df3 100644
--- a/cmd/state.go
+++ b/cmd/state.go
@@ -181,3 +181,40 @@ func (state *State) RemoveLegacySTH(logInfo *certspotter.LogInfo) error {
 	os.Remove(filepath.Join(state.path, "legacy_sths"))
 	return err
 }
+func (state *State) LockFilename() string {
+	return filepath.Join(state.path, "lock")
+}
+func (state *State) Lock() (bool, error) {
+	file, err := os.OpenFile(state.LockFilename(), os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0666)
+	if err != nil {
+		if os.IsExist(err) {
+			return false, nil
+		} else {
+			return false, err
+		}
+	}
+	if _, err := fmt.Fprintf(file, "%d\n", os.Getpid()); err != nil {
+		file.Close()
+		os.Remove(state.LockFilename())
+		return false, err
+	}
+	if err := file.Close(); err != nil {
+		os.Remove(state.LockFilename())
+		return false, err
+	}
+	return true, nil
+}
+func (state *State) Unlock() error {
+	return os.Remove(state.LockFilename())
+}
+func (state *State) LockingPid() int {
+	pidBytes, err := ioutil.ReadFile(state.LockFilename())
+	if err != nil {
+		return 0
+	}
+	pid, err := strconv.Atoi(string(bytes.TrimSpace(pidBytes)))
+	if err != nil {
+		return 0
+	}
+	return pid
+}