Not only do membrane proteins represent a substantial fraction of the information in a genome, but also they are responsible for many essential biological functions, some of which are unique (e.g., as membrane transporters). As a result, some mutations in genes for membrane proteins cause human diseases. Thus, there are many reasons that the structural biology of membrane proteins is of considerable interest. However, because membrane proteins are not soluble in aqueous solution, only a few examples have been amenable to experimental structural analysis with X-ray crystallography and solution nuclear magnetic resonance (NMR) spectroscopy. The relatively few structures of helical membrane proteins that have been determined provide an initial view of the design principles involved in the use of helices as structural and functional elements, setting the stage for the evaluation of new structures of membrane transporters and other membrane proteins. NMR spectroscopy is an extraordinarily powerful method for describing the structure and dynamics of proteins and other biopolymers. Although there have been some initial successes, further development is needed for it to be generally applicable to membrane proteins.