Cryptocurrencies and the underlying distributed ledger technology have exploded into public consciousness over the last few years, with devotees insisting that the technology will revolutionise financial transactions and ownership data. This column identifies a ‘blockchain trilemma’ whereby no ledger can fully satisfy the three desirable properties of decentralisation, correctness, and cost-efficiency. It further explains how distributed ledgers enhance competition but introduce costs above and beyond the well-known electricity costs.