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Ventricular fibrillation is the principal cause of sudden cardiac arrest and the electrical defibrillation is often the only effective therapy. A very interesting question is represented by the electric parameters of defibrillation shock. Today, monophasic waveform is widely used in Europe and in the United States, but, recently, the Food and Drug Administration grants approval for an automatic external defibrillator (AED) producing a biphasic pulse. In this review we discuss about the effectiveness and the safety of biphasic waveform, by examining a series of human studies between 1982 and 1999. We have found that available data are often incomplete, unclear, dishomogeneous and, consequently, difficult to compare. Furthermore, among the authors there is no concordance about the meaning of "safety", "effectiveness", "success", "equivalence" and "superiority" of biphasic versus monophasic shock: however, biphasic shock, that uses a lower energy level, seems to reduce post-defibrillation heart damage. Due to the lack of homogeneous studies it is not possible to state which kind of signal is more reliable, even if some clinical reports and experimental data seem to tribute to the biphasic waveform a better therapeutic effectiveness and safety. By examining the current scientific literature, we conclude that further studies have to be performed to definitively validate the use of biphasic shock.