Nearly half a century ago, Hvidt and Linderstrøm-Lang established amide hydrogen (NH) isotope exchange as a fruitful probe of structure and dynamics in proteins (1–4). Rapid growth in the use of NH exchange over the last 15 yr has followed from advances in both technology (e.g., multidimensional nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS) and rapid-mixing methodologies) and in our understanding of how NH exchange in proteins is slowed relative to exchange in small peptides (5–11). Most applications of NH exchange involve one of the two major types of exchange experiments. These are distinguished by the population of native protein: the first and most popular type of experiment involves native protein alone, and the second type is used to follow exchange during the process of folding or unfolding. For practical reasons, almost all exchange experiments in proteins focus on the NHs of the peptide backbone. This chapter focuses on the exchange of these NHs from the native protein.