Overview
Proof of Work, or PoW, is the original consensus mechanism, used by Bitcoin, where miners compete to solve cryptographic puzzles to validate transactions and create new blocks. The computational effort required makes dishonest history rewriting prohibitively expensive. PoW was the first mechanism to achieve decentralized agreement without trust in any party.
How It Works
Miners repeatedly vary a small field in the block header, hashing it until the result is below a network-adjustable target. Finding a valid solution requires a large amount of computing power proportional to the total hash rate. The winning miner broadcasts the block, other nodes verify it in milliseconds, and the difficulty adjusts every couple of weeks to keep block times stable as more machines join.
Why It Matters
PoW established the security model of permissionless money: an attacker must outspend the entire network in energy and hardware to rewrite history. Its main costs are energy consumption and hardware centralization among large mining operations. Even after Ethereum moved to proof of stake, PoW remains the benchmark for evaluating the security and trade-offs of other consensus designs.
Related Concepts
PoW is executed by Miners, who assemble Transactions into Blocks. It stands in direct contrast to Proof of Stake, and the security it provides is what Hash functions make possible.