Proposer-Builder Separation (PBS) is a design pattern that decouples block proposing from block building to improve decentralization, fairness, and MEV extraction transparency. Originally deployed in Ethereum via MEV-Boost, PBS enables specialized builders to create optimized blocks, which proposers select without needing to analyze transaction order. While PBS reduces validator burden and risk of centralization, it introduces new challenges like censorship and trust in relays. Efforts are ongoing to develop in-protocol PBS for native enforcement and broader security. As MEV becomes more widespread, PBS is likely to be adopted across multiple blockchain ecosystems.