X-ray lasers

The implications of an X-ray laser for science and technology are briefly examined, taking into account the possibility of revolutionary advances concerning the detailed study of biological structure and processes. There are, however, a number of difficult problems which would have to be solved for the development of an X-ray laser. One of these is related to the opaqueness of cold matter to all X-ray wavelengths. Approaches which might make it possible to develop an X-ray laser in spite of the great difficulties are discussed, giving attention to pumping requirements, atomic states for suitable transitions, population-inverting schemes, and free-free X-ray lasers.