Unit commitment is an operation scheduling function, which is sometimes called predispatch. In the overall hierarchy of generation resources management, the unit commitment function fits between the economic dispatch and the maintenance and production scheduling. Unit commitment schedules the on-and-off times of the generating units, and calculates the minimum cost hourly generation schedule while ensuring that start-up and shutdown rates, minimum up and minimum down times are considered. Systems with hydrostorage capability either with pumped hydrostations or with reservoirs on rivers usually require 1 week horizon times. On the other hand, a system with no "memory" devices and few dynamic components can use much shorter horizon times. Most unit commitment programs operate discretely in time, at 1 h intervals. Systems with short horizon times can successfully deal with time increments as small as a few minutes. The optimal operation and planning of power systems involve consideration of economy operation, system security, and start-up or downtime issues.