diff --git a/README.md.upk b/README.md.upk
new file mode 100644
index 0000000..73338ac
Binary files /dev/null and b/README.md.upk differ
diff --git a/src/greedy_packer.rs b/src/greedy_packer.rs
index 3b77d13..b1875e7 100644
--- a/src/greedy_packer.rs
+++ b/src/greedy_packer.rs
@@ -12,7 +12,7 @@ pub fn pack(
     let mut rans_coder = RansCoder::new(config);
     let mut state = lz::CoderState::new(config);
 
-    let mut pos = 0;
+    let mut pos = config.dictionary_size;
     while pos < data.len() {
         if let Some(ref mut cb) = progress_callback {
             cb(pos);
diff --git a/src/lib.rs b/src/lib.rs
index c3e4cd5..ec0e591 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -71,6 +71,9 @@ pub struct Config {
     pub max_offset: usize,
     /// The maximum match length value to encode when compressing.
     pub max_length: usize,
+
+    /// Size of dictionary at the beginning of data (how many bytes to skip when compressing.)
+    pub dictionary_size: usize,
 }
 
 impl Default for Config {
@@ -92,6 +95,8 @@ impl Default for Config {
 
             max_offset: usize::MAX,
             max_length: usize::MAX,
+
+            dictionary_size: 0,
         }
     }
 }
diff --git a/src/main.rs b/src/main.rs
index ba4aff7..19460de 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -16,6 +16,7 @@ fn main() -> Result<()> {
     let mut level = 2;
     let mut infile: Option<PathBuf> = None;
     let mut outfile: Option<PathBuf> = None;
+    let mut dictionary: Option<PathBuf> = None;
     let mut max_unpacked_size = 512 * 1024 * 1024;
 
     let mut parser = lexopt::Parser::from_env();
@@ -74,6 +75,7 @@ fn main() -> Result<()> {
                 process::exit(0);
             }
             Long("max-unpacked-size") => max_unpacked_size = parser.value()?.parse()?,
+            Long("dictionary") => dictionary = Some(parser.value()?.try_into()?),
             Value(val) if infile.is_none() => infile = Some(val.try_into()?),
             Value(val) if outfile.is_none() => outfile = Some(val.try_into()?),
             _ => return Err(arg.unexpected().into()),
@@ -94,6 +96,15 @@ fn main() -> Result<()> {
             data.reverse();
         }
 
+        if let Some(dictionary) = dictionary {
+            let mut dict = vec![];
+            File::open(dictionary)?.read_to_end(&mut dict)?;
+            config.dictionary_size = dict.len();
+            // prepend dict
+            dict.append(&mut data);
+            data = dict;
+        }
+
         #[cfg(feature = "terminal")]
         let mut packed_data = {
             let mut pb = pbr::ProgressBar::on(std::io::stderr(), data.len() as u64);
diff --git a/src/parsing_packer.rs b/src/parsing_packer.rs
index 9ab0f75..7769ef7 100644
--- a/src/parsing_packer.rs
+++ b/src/parsing_packer.rs
@@ -137,7 +137,7 @@ fn parse(
     }
     add_arrival(
         &mut arrivals,
-        0,
+        encoding_config.dictionary_size,
         Arrival {
             parse: None,
             state: lz::CoderState::new(encoding_config),
@@ -148,7 +148,7 @@ fn parse(
 
     let cost_counter = &mut CostCounter::new(encoding_config);
     let mut best_per_offset = HashMap::new();
-    for pos in 0..data.len() {
+    for pos in encoding_config.dictionary_size..data.len() {
         let match_length = |offset: usize| {
             data[pos..]
                 .iter()