Abstract: Valvular insufficiency can limit the heart's effectiveness as a pump and result in heart failure. The most common treatment for severely dysfunctional valves is surgical implantation of a mechanical or bioprosthetic heart valve. While mechanical heart valves have poor flow characteristics and require life long anticoagulation to prevent thrombosis, tissue valves better simulate natural blood flow and eliminate the need for anticoagulants. However, degeneration and calcification limit tissue valve durability to 5-15 years. Nitinol's (nickel-titanium) biocompatibility, and superelastic and shape-memory properties make thin film nitinol a desirable candidate to use as a heart valve leaflet. Such a valve would be unlikely to develop calcification or thrombosis. Additionally, the shape memory characteristics of nitinol could eliminate the need for surgery altogether by allowing for the development of a catheter-placed heart valve. To test the feasibility of using thin film nitinol as a heart valve leaflet, the transition temperatures, mechanical properties, and biocompatibility were assessed. Thin film nitinol was then incorporated into several prosthetic surgically placed valves employing 'semilunar', 'butterfly' and 'bileaflet' designs. The valves were tested under steady and pulsatile flow conditions to determine transvalvular gradients, leakage, and durability. Flow visualization techniques were used to qualitatively analyze flow patterns around a thin film nitinol heart valve. Structural modeling of the leaflet and the valve design was performed to optimize the valve configuration to minimize leaflet stress and deflection. High breakdown potentials (E_bd) from electrochemical corrosion tests and lack of adverse reaction seen from in vivo experiments indicate thin film nitinol is biocompatible in blood contacting applications and suitable for use as a heart valve leaflet. Under hydrodynamic testing conditions, bileaflet valve designs with thin film nitinol exhibited superior durability over semilunar and butterfly valve designs. Transvalvular gradients of the thin film nitinol bileaflet valve were comparable to gradients obtained from commercially available heart valves. Visualization of the leaflets and flow under pulsatile conditions reveal that flexible leaflets reduce high-flow jets and that the valve design prevents areas of stagnant flow.