diff --git a/db/installed.go b/db/installed.go
new file mode 100644
index 0000000..14f3a3a
--- /dev/null
+++ b/db/installed.go
@@ -0,0 +1,70 @@
+package db
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+
+	"github.com/pkg/errors"
+	"mcquay.me/fs"
+	"mcquay.me/pm"
+)
+
+const in = "var/lib/pm/installed.json"
+
+// AddInstalled adds m to the installed package database.
+func AddInstalled(root string, m pm.Meta) error {
+	db, err := loadi(root)
+	if err != nil {
+		return errors.Wrap(err, "loading installed db")
+	}
+	db[m.Name] = m
+	return savei(root, db)
+}
+
+// IsInstalled checks if m is in the installed package database.
+func IsInstalled(root string, m pm.Meta) (bool, error) {
+	db, err := loadi(root)
+	if err != nil {
+		return false, errors.Wrap(err, "loading installed db")
+	}
+
+	_, r := db[m.Name]
+	return r, nil
+}
+
+func loadi(root string) (pm.Installed, error) {
+	r := pm.Installed{}
+	dbn := filepath.Join(root, in)
+
+	if !fs.Exists(dbn) {
+		return r, nil
+	}
+
+	f, err := os.Open(dbn)
+	if err != nil {
+		return r, errors.Wrap(err, "open")
+	}
+
+	if err := json.NewDecoder(f).Decode(&r); err != nil {
+		return r, errors.Wrap(err, "decoding db")
+	}
+
+	return r, nil
+}
+
+func savei(root string, db pm.Installed) error {
+	f, err := os.Create(filepath.Join(root, in))
+	if err != nil {
+		return errors.Wrap(err, "create")
+	}
+	enc := json.NewEncoder(f)
+	enc.SetIndent("", "\t")
+	if err := enc.Encode(&db); err != nil {
+		return errors.Wrap(err, "decoding db")
+	}
+	if err := f.Close(); err != nil {
+		return errors.Wrap(err, "close db")
+	}
+	return nil
+}
diff --git a/installed.go b/installed.go
new file mode 100644
index 0000000..50ebdbd
--- /dev/null
+++ b/installed.go
@@ -0,0 +1,4 @@
+package pm
+
+// Installed tracks installed packages.
+type Installed map[Name]Meta
diff --git a/pkg/install.go b/pkg/install.go
index c932832..f907342 100644
--- a/pkg/install.go
+++ b/pkg/install.go
@@ -60,6 +60,13 @@ func Install(root string, pkgs []string) error {
 	}
 
 	for _, m := range ms {
+		already, err := db.IsInstalled(root, m)
+		if err != nil {
+			return errors.Wrapf(err, "is installed %v", m.Name)
+		}
+		if already {
+			return errors.Errorf("%v already installed!", m.Name)
+		}
 		if err := verifyManifestIntegrity(root, m); err != nil {
 			return errors.Wrap(err, "verifying pkg integrity")
 		}
@@ -85,6 +92,10 @@ func Install(root string, pkgs []string) error {
 		if err := os.Remove(filepath.Join(cacheDir, m.Pkg())); err != nil {
 			return errors.Wrapf(err, "cleaning up pkg %v", m.Pkg())
 		}
+
+		if err := db.AddInstalled(root, m); err != nil {
+			return errors.Wrapf(err, "adding ", m.Name)
+		}
 	}
 	return nil
 }