Abstract C1, the multimolecular protease that triggers the classical pathway of complement, has a major role in the host defense against pathogens. It also participates in other biological functions, such as immune tolerance, owing to the ability of its binding subunit, C1q, to recognize abnormal structures from self, including apoptotic cells. Structural biology has been used over the past few years to elucidate the structure of its three subunits: C1q, C1r and C1s. These new advances have led to a comprehensive, three-dimensional model of C1 and provide insights into the mechanisms underlying its activation and the extraordinarily versatile recognition properties of its C1q subunit.