A block header is the compact “front page” of a blockchain block, containing key metadata that identifies the block and links it to the chain. In proof-of-work systems, miners repeatedly hash the block header, not the entire block, to find a valid hash and earn mining rewards.
What a block header contains
A full block may include many transactions, but the header summarizes that content and the block’s position in history. On Bitcoin, the header is famously 80 bytes and includes fields such as the previous block’s hash (which creates the chain), a Merkle root (a single hash representing all transactions in the block), a timestamp, a difficulty target, a nonce, and a version value. Because the header commits to the transaction set via the Merkle root, changing even one transaction would change the Merkle root and therefore change the block header hash.
How headers enable proof of work and efficient validation
In proof of work, miners vary the nonce and, in practice, other adjustable parts of the block, to produce different header hashes until one meets the network’s difficulty requirement. Hashing the header is far more efficient than hashing the entire block repeatedly, while still securely “locking in” the block’s contents.
Block headers also support lightweight verification. For example, Simplified Payment Verification (SPV) wallets can download and verify a chain of headers, then request a Merkle proof that a specific transaction is included in a particular block, without downloading every transaction in every block.
Understanding block headers matters because they are central to how many blockchains link blocks together, enforce immutability, and make mining and verification scalable across the crypto ecosystem.