Implantable cardioverter defibrillators (ICDs) are available with independently programmable duration and tilt of the shock pulse waveform. Manufacturers do not, however, commonly advise how these parameters can be programmed for optimal clinical benefit. From theoretical considerations, the author recommends programming both parameters based on the measured lead system resistance R into which the shock is delivered. Assuming that the defibrillation pulse decline below the defibrillation threshold rheobase is undesirable because of the possibility of refibrillation. Mathematical relationships expressing optimal pulse duration and tilt as functions of the output time constant can be derived that are valid for monophasic pulses and the first phase of biphasic pulses. Two ICD manufacturers provide for programmable tilt (Medtronic GEM III, atrial channel) or both tilt T and pulse duration PD. (St. Jude Medical newest devices). Considering its output capacitance, it is recommended that the Medtronic Gem III should be programmed for T = 50% when R < 75 omega and 65% when R < 38 omega. The author considers programming tilt to 30% or 40% useless in clinical conditions. By the same reasoning, he recommends that the newer St. Jude Medical ICDs should be programmed to T = 50% if R < 75 omega and 60% if R < 41 omega, and PD = 5.5, 5.0, 4.5, 4.0, 3.5, and 3.0 ms for R < 75, 73, 62, 51, 40, and 32 omega, respectively. PD = 6 ms was considered clinically unrealistic. Programmable shock pulse duration and tilt are useful in optimizing defibrillation, but it is suggested that this can best be accomplished by programming these parameters with the guidance of theory as described in this discussion.