Proposer/builder separation (PBS) is a blockchain design feature that divides the roles of block proposers and block builders. Block proposal is the action of submitting a block of transactions for the approval of network validators, while block building is the action of transaction ordering. By separating these two actions, the process of completing each task is simplified, allowing actors to specialize in one or the other. This separation is important for the decentralization of Ethereum because it minimizes the compute overhead required to become a validator, thereby lowering the barrier to entry and encouraging a more diverse group of participants. PBS also aligns with the goal of The Merge to move Ethereum towards a more modular and decentralized future.