Prophylaxis, or the routine scheduled replacement of clotting factor concentrate in patients with hemophilia, greatly decreases episodes of joint hemorrhage and effectively prevents the development of chronic joint arthropathy and disability. Despite clear evidence of its effectiveness and the fact that it is recommended by international authorities, prophylaxis still is not widely accepted in the hemophilia community. In the United States, approximately half of children with severe hemophilia A and one third of those with hemophilia B are on prophylactic regimens. Factors limiting acceptance include the need for venous access, factor availability, poor acceptance of injection therapy, safety concerns, cost, and a perceived lack of need. Questions remaining to be answered include the reasons for inter-individual variability in bleeding patterns, predictors for and reversibility of joint damage, and the optimal regimen for prophylaxis. A need exists for validated and standardized outcome measures in future research, as well as for research into factors that affect families' adherence to prophylactic regimens for their children.