Operating reserves are the front lines in the defense of electric systems against major generation and transmission outages. Operating reserves are provided by generating units that can increase their output rapidly to restore the system to generation/load balance after a major disturbance occurs. We examine the functions of and the requirements for operating reserves, and the data and analysis that support minimum-operating-reserve requirements. We obtained data on the number and severity of generator outages for three large US electrical systems, including a power pool, a regional reliability council, and a utility. These data show a range of 15 to 40 major (> 500 MW) outages a year for systems ranging in size from 20,000 to 48,000 MW of peak demand. Finally, we discuss several emerging issues related to the underlying technical support for minimum reserve requirements, alternative ways to pay for these reserves, the data needed to support the analysis of these requirements, and the mix of functions that are currently included within operating reserves.