The early use of AC waveforms for ventricular defibrillation is briefly recounted. Research on the efficacy of damped sinusoidal, truncated, and biphasic waveforms is reviewed. It is noted that damped sinusoidal and monophasic truncated exponential waveforms have been used successfully for decades in clinical settings and the further improvements can be made by altering the applied waveforms. Studies in animals suggest the superiority of biphasic over monophasic waveforms; the causes for these improvements have not yet been completely elucidated. Results indicate that the response of the myocardium to electric shocks is complex.