The good quality of life now anticipated by individuals with haemophilia A and B can be drastically altered by the unpredictable development of an inhibitor, a polyclonal high-affinity IgG antibody directed against factor VIII or IX. This paper discusses our current state of knowledge about inhibitors within the context of: (1) the extent and nature of the problem, including its incidence and prevalence, diagnostic options, and the characteristics of the individual immunologic response; (2) possible underlying aetiological factors, both inherited and situational; and (3) existing and future therapeutic options for both the interim treatment of haemorrhage and the ultimate restoration of normal clotting factor recovery and survival. Current and future strategies for immune tolerisation are also reviewed.