The design of input power couplers represents one of the most important challenges of accelerators that use super-conducting RF technology. These devices must fulfil several functions while being subject to mechanical, electromagnetic, vacuum and cryogenic constraints. The rapidly increasing number of projects, planned or under construction, which propose to use super-conducting cavities has prompted developments in power couplers for both CW and pulsed applications. Amongst the projects for which couplers have, or are being, developed one finds VUV and X-ray free electron lasers (based on self-amplified spontaneous emission), spallation neutron sources (SNS), energy recovery linacs, and high energy colliders. We will review the design requirements and performances obtained for several of these couplers. Particular attention will be paid to the couplers which have been used on the TESLA Test Facility at DESY, the variations of this coupler which are under consideration for future synchrotron light sources and the coupler chosen for use on the SNS.