The study of the three-dimensional structures of biologically important macromolecules and macromolecular complexes at the atomic level in order to gain understanding of the relationship between structure and function. This information provides unique and valuable insights into the evolution of proteins and the biological effects of genetic defects. Three-dimensional structures of macromolecules provide the framework upon which novel chemical, functional, and theoretical studies may be based, and they greatly facilitate rational drug design efforts. Investigators in this area employ state-of-the-art X-ray diffraction instrumentation, a 600 MHz NMR spectrometer, and a variety of electron paramagnetic resonance (EPR) spectrometers for macromolecular structure and dynamics determinations.