The immersed boundary method was first introduced to study the fluid dynamics of heart valves. Around the valves has grown a computer model of the heart as a whole, including all four cardiac chambers, all four valves, and the great vessels that connect the heart to the rest of the circulation. This computer model employs a fiber-fluid representation of the heart. In each time step, forces are generated in elastic and contractile fibers. These forces are then allowed to act on a uniform cubic lattice, on which the equations of fluid dynamics are solved. Finally, the fibers move at the local fluid velocity. Neither the fluid motion nor the cardiac tissue motion is assumed known in advance. Instead, their simultaneous equations of motion are solved. For this reason, the immersed boundary method is particularly suitable for the computer simulation of diseases affecting the mechanical function of the heart or its valves, and also for the computer assisted design of devices such as prosthetic cardiac valves that interact with the heart and with the blood flow that occurs in the cardiac chambers.