Many illumination tasks require large area light sources, e.g. LCD backlighting or general lighting. Depending on the application these sources have to fulfill criteria concerning uniformity, angular distribution and color of light output, brightness, efficiency, flatness and cost. After discussing established solutions with regard to these requirements, light guides which incorporate light sources like light emitting diodes or thin fluorescent lamps in cavity like recesses are introduced. The advantages of this scheme are flatness, scalability in area and reliance on established sources. This will be demonstrated for large area light sources utilizing high power LEDs or thin fluorescent lamps.