diff --git a/Cargo.toml b/Cargo.toml
index 5255f6c..d132488 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -6,3 +6,7 @@ edition = "2021"
 [dependencies]
 thiserror = "1.0"
 clap = { version = "4.4", features = ["derive"] }
+fs-err = "2.11"
+
+[dev-dependencies]
+tempfile = "3.8"
diff --git a/src/main.rs b/src/main.rs
index a44e53c..c290fc9 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -2,35 +2,112 @@ mod tag_engine;
 
 use std::collections::BTreeSet;
 use std::error::Error;
-use clap::Parser;
+use clap::{Parser, Subcommand};
+use fs_err as fs;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum CommandError {
+    #[error("Failed to rename {from} to {to}: {source}")]
+    Rename {
+        from: String,
+        to: String,
+        source: std::io::Error,
+    },
+    #[error("Failed to parse tags in {file}: {source}")]
+    Parse {
+        file: String,
+        source: tag_engine::ParseError,
+    },
+}
 
 #[derive(Parser)]
 #[command(author, version, about, long_about = None)]
 struct Cli {
-    /// Files to process
-    #[arg(required = true)]
-    files: Vec<String>,
+    #[command(subcommand)]
+    command: Commands,
 }
 
-fn list_tags(files: &[String]) -> Result<Vec<String>, Box<dyn Error>> {
+#[derive(Subcommand)]
+enum Commands {
+    /// List all unique tags
+    List {
+        /// Files to process
+        files: Vec<String>,
+    },
+    /// Add tags to files
+    Add {
+        /// Tags to add
+        #[arg(required = true)]
+        tags: Vec<String>,
+        /// Files to process
+        #[arg(required = true)]
+        files: Vec<String>,
+    },
+}
+
+fn list_tags(files: &[String]) -> Result<Vec<String>, CommandError> {
     let mut unique_tags = BTreeSet::new();
 
     for file in files {
-        if let Ok((_, tags, _)) = tag_engine::parse_tags(file) {
-            unique_tags.extend(tags);
+        match tag_engine::parse_tags(file) {
+            Ok((_, tags, _)) => unique_tags.extend(tags),
+            Err(e) => return Err(CommandError::Parse {
+                file: file.to_string(),
+                source: e,
+            }),
         }
     }
 
     Ok(unique_tags.into_iter().collect())
 }
 
+fn add_tags_to_file(file: &str, new_tags: &[String]) -> Result<(), CommandError> {
+    let (base, current_tags, ext) = tag_engine::parse_tags(file).map_err(|e| CommandError::Parse {
+        file: file.to_string(),
+        source: e,
+    })?;
+
+    let merged_tags = tag_engine::add_tags(current_tags, new_tags.to_vec());
+    // Preserve the original directory
+    let parent = std::path::Path::new(file).parent()
+        .map(|p| p.to_string_lossy().to_string())
+        .unwrap_or_default();
+    
+    let new_filename = tag_engine::serialize_tags(&base, &merged_tags, &ext);
+    let new_path = if parent.is_empty() {
+        new_filename
+    } else {
+        format!("{}/{}", parent, new_filename)
+    };
+
+    // Only rename if the name would actually change
+    if file != new_path {
+        fs::rename(file, &new_path).map_err(|e| CommandError::Rename {
+            from: file.to_string(),
+            to: new_path,
+            source: e,
+        })?;
+    }
+
+    Ok(())
+}
+
 fn main() -> Result<(), Box<dyn Error>> {
     let cli = Cli::parse();
     
-    let tags = list_tags(&cli.files)?;
-    
-    for tag in tags {
-        println!("{}", tag);
+    match cli.command {
+        Commands::List { files } => {
+            let tags = list_tags(&files)?;
+            for tag in tags {
+                println!("{}", tag);
+            }
+        }
+        Commands::Add { tags, files } => {
+            for file in files {
+                add_tags_to_file(&file, &tags)?;
+            }
+        }
     }
 
     Ok(())
@@ -39,6 +116,18 @@ fn main() -> Result<(), Box<dyn Error>> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use tempfile::TempDir;
+
+    fn create_test_file(dir: &TempDir, name: &str) -> Result<String, Box<dyn Error>> {
+        let path = dir.path().join(name);
+        if let Some(parent) = path.parent() {
+            fs::create_dir_all(parent)?;
+        }
+        fs::write(&path, "")?;
+        Ok(path.to_str()
+            .ok_or("Invalid path")?
+            .to_string())
+    }
 
     #[test]
     fn test_list_tags_empty() {
@@ -65,12 +154,72 @@ mod tests {
     }
 
     #[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"]);
+    fn test_add_tags_to_file() -> Result<(), Box<dyn Error>> {
+        let tmp_dir = TempDir::new()?;
+        let file = create_test_file(&tmp_dir, "test.txt")?;
+        
+        // Verify file was created
+        let initial_path = tmp_dir.path().join("test.txt");
+        assert!(initial_path.exists(), "Initial test file was not created");
+        
+        add_tags_to_file(&file, &vec!["tag1".to_string(), "tag2".to_string()])?;
+        
+        // Verify original is gone and new exists
+        assert!(!initial_path.exists(), "Original file still exists after rename");
+        let new_path = tmp_dir.path().join("test -- tag1 tag2.txt");
+        assert!(new_path.exists(), "Tagged file was not created");
+        Ok(())
+    }
+
+    #[test]
+    fn test_add_tags_to_existing_tags() -> Result<(), Box<dyn Error>> {
+        let tmp_dir = TempDir::new()?;
+        let file = create_test_file(&tmp_dir, "test -- existing.txt")?;
+        
+        // Verify file was created
+        let initial_path = tmp_dir.path().join("test -- existing.txt");
+        assert!(initial_path.exists(), "Initial test file was not created");
+        
+        add_tags_to_file(&file, &vec!["new".to_string()])?;
+        
+        // Verify original is gone and new exists
+        assert!(!initial_path.exists(), "Original file still exists after rename");
+        let new_name = tmp_dir.path().join("test -- existing new.txt");
+        assert!(new_name.exists(), "Tagged file was not created");
+        Ok(())
+    }
+
+    #[test]
+    fn test_add_duplicate_tags() -> Result<(), Box<dyn Error>> {
+        let tmp_dir = TempDir::new()?;
+        let file = create_test_file(&tmp_dir, "test -- tag1.txt")?;
+        
+        // Verify file was created
+        let initial_path = tmp_dir.path().join("test -- tag1.txt");
+        assert!(initial_path.exists(), "Initial test file was not created");
+        
+        add_tags_to_file(&file, &vec!["tag1".to_string()])?;
+        
+        // Original should still exist since no change was needed
+        assert!(initial_path.exists(), "Original file should still exist");
+        Ok(())
+    }
+
+    #[test]
+    fn test_add_tags_nested_path() -> Result<(), Box<dyn Error>> {
+        let tmp_dir = TempDir::new()?;
+        let file = create_test_file(&tmp_dir, "nested/path/test.txt")?;
+        
+        // Verify file was created
+        let initial_path = tmp_dir.path().join("nested/path/test.txt");
+        assert!(initial_path.exists(), "Initial test file was not created");
+        
+        add_tags_to_file(&file, &vec!["tag1".to_string()])?;
+        
+        // Verify original is gone and new exists in same directory
+        assert!(!initial_path.exists(), "Original file still exists after rename");
+        let new_path = tmp_dir.path().join("nested/path/test -- tag1.txt");
+        assert!(new_path.exists(), "Tagged file was not created in original directory");
+        Ok(())
     }
 }
diff --git a/src/tag_engine.rs b/src/tag_engine.rs
index 327fa09..cb956a2 100644
--- a/src/tag_engine.rs
+++ b/src/tag_engine.rs
@@ -34,7 +34,13 @@ pub fn validate_tag(tag: &str) -> Result<(), TagError> {
 pub const TAG_DELIMITER: &str = " -- ";
 
 pub fn parse_tags(filename: &str) -> Result<(String, Vec<String>, String), ParseError> {
-    let parts: Vec<&str> = filename.split(TAG_DELIMITER).collect();
+    // Get the file name without the path
+    let file_name = std::path::Path::new(filename)
+        .file_name()
+        .map(|s| s.to_string_lossy().into_owned())
+        .unwrap_or_else(|| filename.to_string());
+
+    let parts: Vec<&str> = file_name.split(TAG_DELIMITER).collect();
     
     if parts.len() > 2 {
         return Err(ParseError::MultipleDelimiters);
@@ -42,24 +48,30 @@ pub fn parse_tags(filename: &str) -> Result<(String, Vec<String>, String), Parse
 
     // Split the first part into base and extension
     let base_parts: Vec<&str> = parts[0].rsplitn(2, '.').collect();
-    let (base_name, extension) = match base_parts.len() {
-        2 => (base_parts[1].to_string(), format!(".{}", base_parts[0])),
-        _ => (parts[0].to_string(), String::new()),
+    let mut extension = match base_parts.len() {
+        2 => format!(".{}", base_parts[0]),
+        _ => String::new(),
     };
+    let base_name = base_parts.last().unwrap_or(&parts[0]).to_string();
     
     let tags = if parts.len() == 2 {
-        let tag_part = parts[1];
-        let tags: Vec<String> = tag_part
-            .split_whitespace()
-            .map(str::to_string)
-            .collect();
-            
-        // Validate each tag
-        for tag in &tags {
-            validate_tag(tag)?;
+        let mut tag_part = parts[1].to_string();
+        
+        // Check if the last tag contains an extension
+        if let Some(last_part) = tag_part.split_whitespace().last() {
+            if let Some(dot_pos) = last_part.rfind('.') {
+                extension = last_part[dot_pos..].to_string();
+                tag_part.truncate(tag_part.len() - extension.len());
+            }
         }
         
-        tags
+        let mut unique_tags = std::collections::HashSet::new();
+        for tag in tag_part.split_whitespace() {
+            validate_tag(tag)?;
+            unique_tags.insert(tag.to_string());
+        }
+        
+        unique_tags.into_iter().collect()
     } else {
         Vec::new()
     };
@@ -216,4 +228,20 @@ mod tests {
             Err(ParseError::InvalidTag(TagError::InvalidChar(':')))
         );
     }
+
+    #[test]
+    fn test_parse_tags_with_path() {
+        let (base, tags, ext) = parse_tags("/tmp/path/to/file.txt -- tag1 tag2").unwrap();
+        assert_eq!(base, "file");
+        assert_eq!(tags, vec!["tag1", "tag2"]);
+        assert_eq!(ext, ".txt");
+    }
+
+    #[test]
+    fn test_parse_tags_with_duplicate_tags() {
+        let (base, tags, ext) = parse_tags("/tmp/.tmpRRop05/test -- tag1 tag1.txt").unwrap();
+        assert_eq!(base, "test");
+        assert_eq!(tags, vec!["tag1"]);
+        assert_eq!(ext, ".txt");
+    }
 }