Regulator of G protein signaling (RGS) proteins accelerate GTP hydrolysis by Gα subunits and thus facilitate termination of signaling initiated by G protein-coupled receptors (GPCRs). RGS proteins hold great promise as disease intervention points, given their signature role as negative regulators of GPCRs—receptors to which the largest fraction of approved medications are currently directed. RGS proteins share a hallmark RGS domain that interacts most avidly with Gα when in its transition state for GTP hydrolysis; by binding and stabilizing switch regions I and II of Gα, RGS domain binding consequently accelerates Gα-mediated GTP hydrolysis. The human genome encodes more than three dozen RGS domain-containing proteins with varied Gα substrate specificities. To facilitate their exploitation as drug-discovery targets, we have taken a systematic structural biology approach toward cataloging the structural diversity present among RGS domains and identifying molecular determinants of their differential Gα selectivities. Here, we determined 14 structures derived from NMR and x-ray crystallography of members of the R4, R7, R12, and RZ subfamilies of RGS proteins, including 10 uncomplexed RGS domains and 4 RGS domain/Gα complexes. Heterogeneity observed in the structural architecture of the RGS domain, as well as in engagement of switch III and the all-helical domain of the Gα substrate, suggests that unique structural determinants specific to particular RGS protein/Gα pairings exist and could be used to achieve selective inhibition by small molecules.