# What are Bitcoin rollups?

Bitcoin rollups are high-performance Bitcoin L2s that enable smart contracts and dapps to use Bitcoin as the secure base layer. Bitcoin rollups extend the capabilities of Bitcoin without changing Bitcoin, unlocking trillions in capital.

Rollups bundle (or ’roll up’) hundreds of transactions into a single Bitcoin transaction. This distributes the Bitcoin transaction fees across everyone in the rollup, making it cheaper for each user.

The transaction data in the rollup is submitted to Bitcoin, but the rollup executes it separately. By submitting transaction data to Bitcoin, rollups inherit Bitcoin's security. This is because once the data is uploaded to Bitcoin, reverting a rollup transaction requires reverting Bitcoin.

There are two different approaches to rollups: optimistic and zero-knowledge. They differ primarily on how this transaction data is submitted to Bitcoin.

## Optimistic rollups

Optimistic rollups are 'optimistic' in the sense that transactions are assumed to be valid but can be challenged if necessary. If an invalid transaction is suspected, a fault-proof is run to see if this has occurred.

{% content-ref url="/pages/u06Os483yTgxwSTHw5Il" %}
[Optimistic rollups](/bvm/l2-scaling-solutions/optimistic-rollups.md)
{% endcontent-ref %}

## ZK rollups

Zero-knowledge rollups use validity proofs, where transactions are computed off-chain, and compressed data is supplied to Ethereum Mainnet as proof of their validity.

{% content-ref url="/pages/S6EUgVDlxRy3ZVaIRq4X" %}
[ZK rollups](/bvm/l2-scaling-solutions/zk-rollups.md)
{% endcontent-ref %}


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