The major challenge in the development of monolithic kW class CW fiber lasers is the efficient conversion of pump
photons into a high brightness laser beam under the constraints of heat management, long term stability and
nonlinearities. This article reviews the interaction of some fiber related aspects as e.g. fiber core composition,
photodarkening and modality, as well as their influence on system complexity and power scalability. Recent work on
active fibers, pump couplers, mode field adaptors and other fiber-optic components will be presented.