> For the complete documentation index, see [llms.txt](https://build.sovryn.com/builder-portal/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://build.sovryn.com/builder-portal/smart-contracts/fastbtc.md).

# FastBTC

FastBTC is the service that facilitates fast transfers from BTC (on the Bitcoin blockchain) to rBTC (on RSK), and vice versa. It consists of two separate systems

* BTC -> RSK FastBTC
* [RSK -> BTC FastBTC (bidirectional FastBTC)](/builder-portal/smart-contracts/fastbtc/rsk-to-btc.md)