Ever since Satoshi Nakamoto first publicly announced bitcoin, its supporters, critics, and skeptics alike have questioned how the protocol would scale as usage increases over time. This question is more important than ever today, as blocks are increasingly full or close to full of transactions. So-called "Layer 2" (L2) protocols such as the Lightning Network have been deployed to take some transaction volume "offchain" but even Lightning needs to use some bitcoin block space. It’s clear that as bitcoin is adopted by more and more of the world’s population (human and machine alike!) more block space will be needed. Another thread of inquiry concerns whether bitcoin’s limited scripting capabilities help or hinder its value as electronic cash. Researchers and inventors have shown that the electronic cash transactions first made possible by bitcoin could be given new form by improving transaction privacy, supporting new types of smart contracts, and even creating entirely new blockchain-based assets.

One of the results of the decade-plus research into scaling and expanding the capabilities of blockchains such as bitcoin is the invention of the validity rollup. Given the observed benefits that validity rollups have for the blockchains that have already implemented them, attention now turns to the question of whether they would be beneficial for bitcoin and existing bitcoin L2 protocols such as Lightning, too. We explore this question by examining validity rollups from several angles, including their history, how they work on a technical level, how they could be built on bitcoin, and what the benefits, costs, and risks of building them on bitcoin might be. We conclude that validity rollups have the potential to improve the scalability, privacy, and programmability of bitcoin without sacrificing bitcoin’s core values or functionality as a peer-to-peer electronic cash system. Given the "trustless" nature of validity rollups as cryptographically-secured extensions of their parent chain, and given bitcoin’s status as the most secure settlement layer, one could even say these protocols are a perfect match for one another.