The recent global interest in cryptocurrencies was triggered by the rise of Bitcoin, which introduced a public ledger called the blockchain to record the history of its transactions. Bitcoin maintains its blockchain through a decentralized peer-to-peer cryptographic protocol that works without any trusted central authority, but it assumes (at least) that a majority of the computing power in the system is held by honest nodes. Cryptographic techniques ensure the integrity of all transactions in the distributed ledger and new entries are appended through a consensus protocol. Many alternative cryptocurrencies have introduced variations of Bitcoin and proposed alternative designs for consensus ledgers. Novel protocols, known as "smart contracts", are constructed on top of the blockchain, achieving guarantees that were not possible before. Today many financial institutions see the disruptive power of this technology and regard it as a promising alternative to their established business practices, not depending on centralized control, eliminating intermediaries, and enabling new businesses.

Several consensus mechanisms are currently under investigation: On the one hand, Bitcoin's consensus protocol, called "Nakamoto consensus", allows anonymous nodes to participate based on a "proof-of-work". On the other hand, traditional Byzantine consensus and BFT protocols play a role in settings where all nodes are known to each other. Many other systems have been proposed and lie somewhere between these extremes. Distributed knowledge, consistency, and reaching consensus among selfish and mutually distrusting nodes are core topics in the theory and practice of distributed computing. Hence, the popularity of decentralized cryptocurrencies and consensus ledgers creates a unique opportunity for the field to explore this nascent domain and to influence it.