The general principles underlying coin mechanisms are considered and analysed, also their relations to coins and tokens. Outstanding historical examples are described, followed by the consideration of some modern applications.The application of coin mechanisms to electricity meters is dealt with. The influence of tariffs and the reasons and justifications of this method of revenue collection are discussed. Essential points of operation and the conditions of optimum performance are considered.Finally the paper deals with frauds, foreign practice, and testing, in relation to electricity meters.