diff --git a/Cargo.toml b/Cargo.toml
index cd1b656..5255f6c 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -5,3 +5,4 @@ edition = "2021"
 
 [dependencies]
 thiserror = "1.0"
+clap = { version = "4.4", features = ["derive"] }
diff --git a/src/main.rs b/src/main.rs
index 2ee2541..a44e53c 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,5 +1,76 @@
 mod tag_engine;
 
-fn main() {
-    println!("Hello, world!");
+use std::collections::BTreeSet;
+use std::error::Error;
+use clap::Parser;
+
+#[derive(Parser)]
+#[command(author, version, about, long_about = None)]
+struct Cli {
+    /// Files to process
+    #[arg(required = true)]
+    files: Vec<String>,
+}
+
+fn list_tags(files: &[String]) -> Result<Vec<String>, Box<dyn Error>> {
+    let mut unique_tags = BTreeSet::new();
+
+    for file in files {
+        if let Ok((_, tags, _)) = tag_engine::parse_tags(file) {
+            unique_tags.extend(tags);
+        }
+    }
+
+    Ok(unique_tags.into_iter().collect())
+}
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let cli = Cli::parse();
+    
+    let tags = list_tags(&cli.files)?;
+    
+    for tag in tags {
+        println!("{}", tag);
+    }
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_list_tags_empty() {
+        let files = Vec::new();
+        let tags = list_tags(&files).unwrap();
+        assert!(tags.is_empty());
+    }
+
+    #[test]
+    fn test_list_tags_single_file() {
+        let files = vec!["document.txt -- tag1 tag2".to_string()];
+        let tags = list_tags(&files).unwrap();
+        assert_eq!(tags, vec!["tag1", "tag2"]);
+    }
+
+    #[test]
+    fn test_list_tags_multiple_files() {
+        let files = vec![
+            "doc1.txt -- tag1 tag2".to_string(),
+            "doc2.txt -- tag2 tag3".to_string(),
+        ];
+        let tags = list_tags(&files).unwrap();
+        assert_eq!(tags, vec!["tag1", "tag2", "tag3"]);
+    }
+
+    #[test]
+    fn test_list_tags_with_invalid() {
+        let files = vec![
+            "valid.txt -- good tag1".to_string(),
+            "invalid.txt -- bad:tag".to_string(),
+        ];
+        let tags = list_tags(&files).unwrap();
+        assert_eq!(tags, vec!["good", "tag1"]);
+    }
 }