Structural biology is the study of the three-dimensional structure of biological important molecules and macromolecules such as carbohydrates, proteins and nucleic acids. The 3D structure of these molecules generally defines their function. By determining their atomic structures, we can gain significant insights into how they function, and why mutations or deletions give rise to particular diseases. Analysis of the atomic-level structure of proteins also allows the rational design of small molecules that manipulates their function and therefore have powerful therapeutic potential. The two main techniques that are used to determine atomic level details of macromolecular structure are X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy. Within our department, these techniques are used to study the function of proteins involved in the mammalian immune response, retroviral assembly, and the structure of cell-surface carbohydrates. Within the last several years, other techniques such as electrospray ionization mass spectrometry (ESI-MS), have also been developed to study macromolecular structure that are not accessible by NMR or X-ray crystallography. This approach is being used within our department to probe nucleic acid and protein-nucleic acid structures in retroviruses.