Site-directed spin labeling has become a popular biophysical tool for the characterization of protein structure, dynamics and conformational change. This method is well suited and widely used to study small soluble proteins, membrane proteins and large protein complexes. Recent advances in site-directed spin labeling methodology have occurred in two areas. The first involves an understanding of the conformations and local dynamics of the spin-labeled sidechain, including the features of proteins that influence electron paramagnetic resonance lineshape. The second advance is the application of pulse techniques to determine long-range distances and distance distributions in proteins. During the past two years, these technical developments have been used to address several important problems concerning the molecular function of proteins.