The world of x-ray sources is divided between large "third generation" synchrotron and the user-friendlier "table-top" sources using the emission from plasmas produced by lasers or electrical discharges. The use of x-ray sources has become more and more widespread in many disciplines of basic research and engineering because x-rays are invaluable tools for imagery, defect analysis, and material composition and structure determination. X-ray sources are now capable of producing ultra short pulses with duration ranging from tens of picoseconds to less than one picosecond, giving access to the time scale of elementary processes such as molecular vibrations, and gaining information about the transition state in reactive biological molecules. This paper describes these new x-ray sources and techniques and reviews the first experiments dedicated to the analysis of atomic motions on this time-scale.