Biological membranes are densely packed with membrane proteins that occupy approximately half of their volume. In almost all cases, membrane proteins in the native state lack the higher-order symmetry required for their direct study by diffraction methods. Despite many technical difficulties, numerous crystal structures of detergent solubilized membrane proteins have been determined that illustrate their internal organization. Among such proteins, class A G protein-coupled receptors have become amenable to crystallization and high resolution X-ray diffraction analyses. The derived structures of native and engineered receptors not only provide insights into their molecular arrangements but also furnish a framework for designing and testing potential models of transformation from inactive to active receptor signaling states and for initiating rational drug design.