Condorcet and Borda first introduced the consensus theory in the eighteenth century when they studied voting systems. Consensus problems have been investigated for a long time in many fields such as economics, sociology, computer science, bioinformatics, collective intelligence, and automation. They are fundamental to developing distributed systems. In recent years, with blockchain's great applications and potential, consensus problems have received significant attention from many researchers. The consensus method allows a blockchain network to function in a distributed system, especially IoT (Internet of Things). Voting consensus has been applied in distributed systems and blockchain, and consensus in distributed systems is also used in blockchain systems. This study reviews the consensus methods in voting, distributed systems, and blockchain.