The genetic deficiency of the C1 inhibitor is responsible for hereditary angioedema (HAE), which is a disease transmitted as an autosomal dominant trait. More than 200 point mutations in the C1 inhibitor gene have been found to be associated with HAE. Patients with this disease suffer from recurrent angioedema, which is mediated by bradykinin derived from activation of the contact system. This system is physiologically controlled at several steps by the C1 inhibitor. In this review, we describe known mechanisms for the development of angioedema in patients with C1 inhibitor deficiency.