Ethereum’s block production looks decentralised on the surface. Thousands of validators propose blocks, attestations flow in, finality follows. But the actual work of assembling a block - deciding which transactions go in, in what order, extracting whatever MEV is available - has largely moved off-chain to a small cluster of specialised builders. That’s Proposer-Builder Separation in practice, and it’s been operating informally through MEV-Boost since 2022.
What PBS Actually Does
The core idea is a division of labour. A builder constructs the most profitable block they can - aggregating transactions, running searcher bundles, ordering everything to maximise fees and MEV. A proposer (the validator) simply picks the highest bid from a set of builders via a relay, signs the block header, and collects the payment. The proposer never sees the full transaction contents before committing, which prevents them from front-running the block they’re about to publish.
This separation exists because validators generally lack the infrastructure and expertise to compete at MEV extraction. Outsourcing construction to specialists increases validator revenue while keeping the proposer’s role simple and accessible to solo stakers running modest hardware.
The Centralisation Problem Underneath
The trade-off is structural. As of mid-2026, a handful of builders - entities like Titan Builder, beaverbuild, and rsync-builder - have consistently dominated block production on Ethereum by share of blocks built. Exact figures shift weekly, but independent monitoring sites like mevboost.pics have shown the top three builders routinely accounting for over 80% of MEV-Boost blocks across extended periods.
That concentration matters because builders control ordering. A builder who handles enough flow can credibly offer exclusive order flow arrangements to large trading desks and protocols, which in turn feeds more flow back to them. It’s a compounding advantage that makes the market harder to enter over time.
Relays compound this further. Relays sit between builders and proposers, validating that the builder’s block is honest before the proposer signs it. They’re trusted intermediaries that validators depend on but have no direct accountability to the protocol.
Enshrined PBS Isn’t Here Yet
Ethereum’s roadmap includes enshrined PBS - moving the separation into the consensus layer itself rather than leaving it to off-chain infrastructure like MEV-Boost. The design work is ongoing, and proposals like attester-proposer separation (APS) and inclusion lists are being researched as tools to limit builder censorship and reduce relay trust requirements.
What those designs can’t fully solve is whether sophisticated builders will still outcompete everyone else once the rules are formalised at the protocol level. Efficiency pressure tends to re-concentrate even markets that were designed to distribute. Whether enshrined PBS changes that dynamic or just moves it somewhere else in the stack remains an open question.