The complement system was traditionally known as an effector arm of humoral immunity. Today we also recognize it as a main element of the innate immune system. In blood and other body fluids complement is a first line of defence against pathogens, because it becomes fully active within seconds. Active complement fragments attach to the invading pathogen to promote opsonization and lysis, triggering a local inflammatory response. This Review focuses on the evolving role of the complement system in the regulation of T-cell responses, from directing the initiation phase, through driving lineage commitment, to regulating the contraction phase.