diff --git a/Cargo.lock b/Cargo.lock
index 9549447..61e4ba9 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -953,9 +953,9 @@ checksum = "adad44e29e4c806119491a7f06f03de4d1af22c3a680dd47f1e6e179439d1f56"
 
 [[package]]
 name = "registry-conformance"
-version = "0.3.0"
+version = "0.4.0"
 source = "sparse+https://integer32llc.github.io/registry-conformance/"
-checksum = "1616afb1bbc576b6aa79fc318f8b0604340945be5765de85954fa52532b4d38e"
+checksum = "d32618611649b0df9be05d325c0ddfa5fb7e088a5e50662a4b7a3722da73f1c7"
 dependencies = [
  "base64",
  "futures",
diff --git a/Cargo.toml b/Cargo.toml
index a7a7370..ea0a153 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -26,7 +26,7 @@ unused_crate_dependencies = "deny"
 
 [workspace.dependencies]
 argh = { version = "0.1.12", default-features = false }
-registry-conformance = { version = "0.3.0", registry = "registry-conformance" }
+registry-conformance = { version = "0.4.0", registry = "registry-conformance" }
 snafu = { version = "0.8.2", default-features = false, features = ["rust_1_65", "std"] }
 tokio = { version = "1.37.0", default-features = false, features = ["macros", "process", "rt-multi-thread"] }
 
diff --git a/conformance/src/main.rs b/conformance/src/main.rs
index b5edee3..a3ec985 100644
--- a/conformance/src/main.rs
+++ b/conformance/src/main.rs
@@ -185,18 +185,21 @@ impl Margo {
         Ok(())
     }
 
-    async fn yank_crate_(&mut self, crate_: &CreatedCrate) -> Result<(), YankError> {
+    async fn yank_crate_(&mut self, crate_: &CreatedCrate, yanked: bool) -> Result<(), YankError> {
         use yank_error::*;
 
-        self.command()
-            .arg("yank")
+        let mut cmd = self.command();
+        cmd.arg("yank")
             .arg("--registry")
             .arg(&self.directory)
             .arg(crate_.name())
-            .args(["--version", crate_.version()])
-            .expect_success()
-            .await
-            .context(ExecutionSnafu)?;
+            .args(["--version", crate_.version()]);
+
+        if !yanked {
+            cmd.arg("--undo");
+        }
+
+        cmd.expect_success().await.context(ExecutionSnafu)?;
 
         Ok(())
     }
@@ -295,7 +298,11 @@ impl Registry for Margo {
     }
 
     async fn yank_crate(&mut self, crate_: &CreatedCrate) -> Result<(), Error> {
-        Ok(self.yank_crate_(crate_).await?)
+        Ok(self.yank_crate_(crate_, true).await?)
+    }
+
+    async fn unyank_crate(&mut self, crate_: &CreatedCrate) -> Result<(), Error> {
+        Ok(self.yank_crate_(crate_, false).await?)
     }
 
     async fn shutdown(self) -> Result<(), Error> {
diff --git a/src/main.rs b/src/main.rs
index 311ca50..dfd8ab2 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -91,6 +91,10 @@ struct YankArgs {
     #[argh(option)]
     registry: PathBuf,
 
+    /// undo a previous yank
+    #[argh(switch)]
+    undo: bool,
+
     /// the version of the crate
     #[argh(option)]
     version: Version,
@@ -284,7 +288,7 @@ fn do_generate_html(_global: &Global, html: GenerateHtmlArgs) -> Result<(), Erro
 
 fn do_yank(_global: &Global, yank: YankArgs) -> Result<(), Error> {
     let r = Registry::open(yank.registry)?;
-    r.yank(yank.name, yank.version)?;
+    r.yank(yank.name, yank.version, !yank.undo)?;
     Ok(())
 }
 
@@ -383,7 +387,6 @@ impl Registry {
             fs::create_dir_all(path).context(CrateDirSnafu { path })?;
         }
 
-        // FUTURE: Add `unyank` subcommand
         // FUTURE: Add `remove` subcommand
         // FUTURE: Stronger file system consistency (atomic file overwrites, rollbacks on error)
 
@@ -412,12 +415,12 @@ impl Registry {
         Err(HtmlError)
     }
 
-    fn yank(&self, name: CrateName, version: Version) -> Result<(), YankError> {
+    fn yank(&self, name: CrateName, version: Version, yanked: bool) -> Result<(), YankError> {
         use yank_error::*;
 
         self.read_modify_write(&name, |index| {
             let entry = index.get_mut(&version).context(VersionSnafu)?;
-            entry.yanked = true;
+            entry.yanked = yanked;
             Ok(())
         })
     }