Hereditary angioedema (HAE) is a rare autosomal dominant disease clinically characterized by recurrent, often unpredictable attacks of subcutaneous and mucosal swelling. Acute episodes are debilitating, painful, disfiguring, and potentially fatal. HAE type I and type II result from a deficiency in the plasma level of functional C1 inhibitor. HAE with normal levels of C1 inhibitor has been recognized. There is evidence that contact activation underlies the recurrent attacks of swelling. This article reviews laboratory parameters to detect contact system activation and implications for diagnosis of HAE and other forms of bradykinin-mediated angioedema.