Electron cyclotron (EC) heating and current drive is a well-established auxiliary heating mechanism for tokamak plasmas, which is also effective in assisting plasma breakdown and controlling disruptive plasma instabilities. ITER requires 20 MW of power at 170-GHz frequency to be coupled into the plasma for EC radio-frequency (rf) applications. Gyrotrons are rf/microwave oscillators capable of delivering high continuous-wave power in the microwave and millimeter-wave frequency range (a few to hundreds of gigahertz). An EC system with 26 gyrotron sources at 170 GHz, with a typical unit power of 1 MW each, and a total installed power capacity of 24 MW is planned for ITER. As a part of the in-kind contributions, the Indian domestic agency is responsible for two sets of EC sources that provide 2 MW (∼8%) of the EC power at 170-GHz frequency. Here, we provide an overview of the gyrotron source system, its basic concepts and main features, design aspects, auxiliary requirements, performance issues, and future research and development goals.