Solid state X-ray detectors based on large-area amorphous semiconductors, such as amorphous silicon or selenium, have been developed. The requirements for various applications in medical diagnosis determine the boundary conditions for different detectors. Key parameters are image-receptor size, spatial resolution, image frequency, signal-to-noise ratio, and long-term stability. Conventional thorax radiographs are 43 x 35 cm(2) in size. Therefore, to replace film, large detectors are required. Mammograms need to display microcalcifications of some 100 mum in detail. Fluoroscopic images are taken at a rate of 30 images/s, or even faster. Moreover, all detectors need to deliver optimum image quality at the lowest tolerable dose. The lifetime should be at least 10 years. In this paper we discuss different types of detectors that could fulfill those needs. Two fundamental concepts are compared: a scintillator coupled to a photodiode and a direct converting semiconductor, both being arranged on an amorphous silicon-switching matrix.