This article gives an overview of recent x-ray diffraction experiments with time resolutions down to 10^-13s. The scientific motivation behind the development is outlined, using examples from solid state physics and biology. The ultrafast resolution may be provided either by fast detectors or short x-ray pulses, and the limitations of both techniques are discussed on the basis of state of the art experiments. In particular, it is shown that with present designs, high time resolution reduces the structural information attainable with high spatial resolution, thereby limiting feasible experiments on the ultrashort time-scale. The first experiment showing subpicosecond conformation changes was recently achieved with simple solids using an ultrafast laser-produced plasma x-ray source. The principles of this experiment are described in detail.