Modern mass spectrometry (MS) is well known for its exquisite sensitivity in probing the covalent structure of macromolecules, and for that reason, it has become the major tool used to identify individual proteins in proteomics studies. This use of MS is now widespread and routine. In addition to this application of MS, a handful of laboratories are developing and using a methodology by which MS can be used to probe protein conformation and dynamics. This application involves using MS to analyze amide hydrogen/deuterium (H/D) content from exchange experiments. Introduced by Linderstøm-Lang in the 1950s, H/D exchange involves using (2)H labeling to probe the rate at which protein backbone amide protons undergo chemical exchange with the protons of water. With the advent of highly sensitive electrospray ionization (ESI)-MS, a powerful new technique for measuring H/D exchange in proteins at unprecedented sensitivity levels also became available. Although it is still not routine, over the past decade the methodology has been developed and successfully applied to study various proteins and it has contributed to an understanding of the functional dynamics of those proteins.