The Internet Computer integrates at the protocol level with the Bitcoin network. A canister smart contract, the Bitcoin canister, holds the complete set of unspent transaction outputs (UTXOs) of the Bitcoin network in an efficiently-queryable form. A new Bitcoin block appearing in the Bitcoin network is immediately ingested and processed. Any canister on the Internet Computer can query the Bitcoin canister to obtain the balance or UTXO set for any Bitcoin address, as well as current transaction fees. Furthermore, a canister can sign a transaction using chain-key ECDSA signatures, an advanced form of threshold ECDSA, and submit the transaction to the Bitcoin network.