Glossary
Block Builder
A specialized actor that constructs Ethereum blocks under PBS — selecting and ordering transactions (including MEV bundles) to maximize the proposer's payout.
Block builders are the supply side of the MEV-Boost market. They ingest the public mempool, private order flow from wallets and searchers, and MEV-Share bundles; they then assemble a candidate block whose tip + MEV value, after their own cut, is paid to the proposer.
Builder concentration is the central decentralization risk of current PBS: a small number of builders (Beaverbuild, Titan, and a few others) produce most Ethereum blocks, and they could in principle censor or collude.