The agreement problem

If thousands of nodes each receive transactions in a slightly different order, how do they all agree on one official history? That's the consensus problem, and it's the core innovation blockchains solve.

Two mechanisms dominate today: Proof of Work and Proof of Stake. Both make it expensive to cheat and reward honest participation.

TermsConsensus

02

Proof of Work

Miners compete to solve a hard hashing puzzle. The winner proposes the next block and earns a reward. Solving takes enormous electricity, so rewriting history would mean out-spending the entire honest network - a 51% attack that's usually uneconomical.

Bitcoin is the best-known Proof-of-Work chain.

TermsProof of Work 51% attack

03

Proof of Stake

Instead of burning electricity, validators lock up (stake) the network's token. The protocol picks validators to propose and attest blocks. Misbehave, and part of your stake is destroyed - 'slashed'.

This uses far less energy. Ethereum switched from Proof of Work to Proof of Stake in 2022.

TermsProof of Stake

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Primary references

Documentation and standards

References added 2026-10-24

Official technical references for further verification. A source link is not a guarantee that every earlier editorial sentence has been reverified; provider specifications may also change over time.

The curriculum predates this reference index. These sources are offered to check technical details; no original publication date has been inferred.