Waves of diffusing electrons and holes are created when a semiconductor is illuminated with a periodically modulated laser beam. When the electrons and holes recombine, they emit infrared photons whose distribution (shown here in a computer simulation) can be measured and used to investigate the presence of defects inside the semiconductor. To find out more about this and other applications of diffusion waves, read Andreas Mandelis's article, which begins on page 29. (Cover image by Lena Nicolaides and Chris Feng.)