Proof-of-stake (PoS) is the second most popular consensus protocol after PoW. Unlike in PoW, instead of buying very expensive computer equipment to compete in a mining race, miners of PoS invest in the coins of the system. Precisely speaking, the term "miners" should be used with regard to PoW only, while the rest of consensus protocols are run by validators. However, nowadays the terms "mining" and "miner" are usually used to describe the process of validating blocks in all cryptocurrencies, and this article follows this trend, Mining does not exactly describe the PoS protocol because there is no process of coin creation. Instead, all the cryptocoins exist from day one, and miners, also called stakeholders, are paid strictly in transaction fees. The more tokens a miner owns, the more chances it has be chosen to create the next block. Peercoin was the first cryptocurrency to implement PoS. Ethereum, which is currently the second largest cryptocurrency by market cap after Bitcoin, planning to change its PoW to PoS in 2018.