diff --git a/internal/check/check.go b/internal/check/check.go
new file mode 100644
index 0000000..e073f58
--- /dev/null
+++ b/internal/check/check.go
@@ -0,0 +1,90 @@
+// SPDX-License-Identifier: Unlicense
+
+package main
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+)
+
+var goSpdxHeader = `// SPDX-License-Identifier: Unlicense`
+var tmplSpdxHeader = `{{/* SPDX-License-Identifier: Unlicense */`
+
+type MissingSPDXError struct {
+	Name string
+}
+
+func (m MissingSPDXError) Error() string {
+	return fmt.Sprintf("missing or incorrect SPDX header: %s", m.Name)
+}
+
+func checkFileSPDX(header string, name string) error {
+	f, err := os.Open(name)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	r := bufio.NewReader(f)
+	pattern := `(?m:^` + regexp.QuoteMeta(header) + `)`
+	matched, err := regexp.MatchReader(pattern, r)
+	if err != nil {
+		return err
+	}
+	if !matched {
+		return MissingSPDXError{Name: name}
+	}
+	return nil
+}
+
+func walkSPDX(header string, extension string) error {
+	var errs []error
+	filepath.WalkDir(".", func(path string, d fs.DirEntry, err error) error {
+		if path != "." && strings.HasPrefix(path, ".") {
+			if d.IsDir() {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		if err != nil {
+			errs = append(errs, err)
+			return nil
+		}
+
+		if filepath.Ext(path) != extension {
+			return nil
+		}
+		errs = append(errs, checkFileSPDX(header, path))
+		return nil
+	})
+	return errors.Join(errs...)
+}
+
+func checkSPDX() error {
+	err := errors.Join(
+		walkSPDX(goSpdxHeader, ".go"),
+		walkSPDX(tmplSpdxHeader, ".tmpl"),
+	)
+	if err != nil {
+		return fmt.Errorf("Check SPDX Headers:\n%w", err)
+	}
+	return nil
+}
+
+func run() error {
+	return checkSPDX()
+}
+
+func main() {
+	err := run()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
diff --git a/state_test.go b/state_test.go
index 3a64cbe..b8e9b0d 100644
--- a/state_test.go
+++ b/state_test.go
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: Unlicense
+
 package gigaparsec_test
 
 import (