The T cell costimulatory pathways are central to regulating immune responses, and targeting these pathways represents one of the most promising approaches for achieving immunotherapy. The molecular structures of costimulation revealed invaluable mechanistic insights underlying costimulatory receptor/ligand specificity, affinity, oligomeric state, and valency, which provided the bases for better manipulation of these signaling pathways. The incredible growth of this field led to identification of new members and unexpected interactions, revealing a complicated regulatory network of immune responses. The advances in structural biology of costimulation will promise unprecedented opportunities for furthering our understanding and therapeutic application of T cell costimulatory pathways.