Polkadot uses the Nominated Proof of Stake as an algorithm to select a set of validators who are elected in each era. There will always be a limited number of these validators (for performance reasons), there are now around 200 and the target number is 1000. In Polkadot, two roles are defined - a nominator and a validator. Validators are responsible for running the consensus algorithm and creating blocks, while nominators support validators with their DOT tokens. Any validator can become a candidate and thus participate in the election to the active set of validators in a given era - there are no requirements for the validator, for example in the form of a minimum stake, because Polkadot is a permissionless blockchain. Active validators are elected once per era (ie every 24 hours). The network selects current validators according to the amount of their stakeholder - which makes sense, because a larger stakeholder means more reputation and trust that the community places in such a validator. Unlike other PoS systems, each validator selected in this way has the same voting power, regardless of the amount of its stake. The nominator can choose up to 16 validators for staking, but he cannot choose how his stake will be divided among them. Stake is distributed among validators using Phragmen's algorithm, the purpose of which is to balance the number of tokens between individual validators. Because every validator has the same voting power regardless of the size of his stake, the reward is the same for each of them.