diff --git a/Cargo.toml b/Cargo.toml
index 9a16802..e833b7e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -41,4 +41,5 @@ log = "0.4"
 
 noodles-bam = "0.49.0"
 noodles-bgzf = "0.25"
+noodles-csi = "0.26"
 noodles-sam = "0.46"
diff --git a/src/index.rs b/src/index.rs
new file mode 100644
index 0000000..89e1a6c
--- /dev/null
+++ b/src/index.rs
@@ -0,0 +1,66 @@
+use std::io;
+use std::path::Path;
+
+use noodles_bam::{self as bam, bai};
+use noodles_csi::{self as csi, index::reference_sequence::bin::Chunk};
+use noodles_sam::{self as sam, alignment::Record};
+
+fn is_coordinate_sorted(header: &sam::Header) -> bool {
+    use sam::header::record::value::map::header::SortOrder;
+
+    if let Some(hdr) = header.header() {
+        if let Some(sort_order) = hdr.sort_order() {
+            return sort_order == SortOrder::Coordinate;
+        }
+    }
+
+    false
+}
+
+pub fn index_bam<P: AsRef<Path>>(file: P) -> io::Result<()> {
+    let mut reader = bam::reader::Builder.build_from_path(file.as_ref())?;
+    let header = reader.read_header()?;
+
+    if !is_coordinate_sorted(&header) {
+        return Err(io::Error::new(
+            io::ErrorKind::InvalidData,
+            "the input BAM must be coordinate-sorted to be indexed",
+        ));
+    }
+
+    let mut record = Record::default();
+
+    let mut builder = csi::index::Indexer::default();
+    let mut start_position = reader.virtual_position();
+
+    while reader.read_record(&header, &mut record)? != 0 {
+        let end_position = reader.virtual_position();
+        let chunk = Chunk::new(start_position, end_position);
+
+        let alignment_context = match (
+            record.reference_sequence_id(),
+            record.alignment_start(),
+            record.alignment_end(),
+        ) {
+            (Some(id), Some(start), Some(end)) => {
+                Some((id, start, end, !record.flags().is_unmapped()))
+            }
+            _ => None,
+        };
+
+        builder.add_record(alignment_context, chunk)?;
+
+        start_position = end_position;
+    }
+
+    let index = builder.build(header.reference_sequences().len());
+    // let index_file = file.as_ref().with_extension("bai");
+
+    // write to index file
+    let stdout = io::stdout().lock();
+    let mut writer = bai::Writer::new(stdout);
+    writer.write_header()?;
+    writer.write_index(&index)?;
+
+    Ok(())
+}
diff --git a/src/main.rs b/src/main.rs
index a377b83..c7531d2 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -6,6 +6,7 @@ use log::LevelFilter;
 use std::path::PathBuf;
 
 mod extract;
+mod index;
 
 #[derive(Parser)]
 #[command(author, version, about, long_about = None)]
@@ -31,6 +32,12 @@ enum Commands {
         #[arg(short = 'b', default_value = "false")]
         isbam: bool,
     },
+
+    /// Index a BAM file
+    Index {
+        /// Bam input file
+        input: PathBuf,
+    },
 }
 
 fn main() {
@@ -64,6 +71,11 @@ fn main() {
             extract::extract(readids, input, *isbam).unwrap();
         }
 
+        Some(Commands::Index { input }) => {
+            info!("'index'  {input:?} ");
+            index::index_bam(input).unwrap();
+        }
+
         // If no subcommand was used, it's a normal top level command
         None => info!("No subcommand was used"),
     }