It is only recently that the extraordinary variety of motions possible for proteins and nucleic acids has been appreciated. In addition, the develop ment of new physical techniques for probing the structure and behavior of macromolecules has allowed characterization of their dynamics for the first time. It seems fitting that X-ray crystallography, which unintentionally contributed to the old "rigid molecule" view, should make an equal contribution to the new picture of the flexible biopolymer. No other physical method presents such a high resolution view of the non-hydrogen atoms in a macromolecule. It has long been held that this view is without any dynamic information and is therefore inherently inferior to information from techniques such as NMR. However, during the past few years it has become clear that much valuable information about the spatial distribution of atomic fluctuations can be obtained by careful analysis of crystal structures at high resolution. X-ray diffraction is thus complementary to spectroscopic and other techniques, which are low in spatial resolution but rich in temporal information. When used in concert, the modern arsenal of physical methods provides not only a general notion of how fast overall processes occur for a given biopolymer but also a detailed map of the vari ous motions undergone by different regions of the molecule in different time regimes. This review focuses on the application of single crystal X-ray diffraction techniques to the study of protein dynamics. First, an overview is presented