Studies on fertilization have involved a variety of investigational techniques from the simple to the very complex. Perhaps the most direct approach has been the observation and photographic recording of the interactions of living gametes in vitro. The purpose of this paper is to review the major contributions that have been made, by means of this technique, to our knowledge of mammalian fertilization, and to examine its advantages and limitations. Some of the events of mammalian fertilization that have been observed in living eggs and are reviewed herein include sperm penetration through the zona pellucida, contact of the fertilizing spermatozoon with the oocyte surface and the subsequent incorporation of the sperm head into the oocyte cytoplasm, the formation and disappearance of the incorporation cone, the gradual incorporation of the sperm flagellum, surface movements of the oocyte after activation, and the formation of the second polar body. One advantage of studying living eggs is the opportunity it affords to witness events as they actually occur and, under favorable circumstances, to observe the whole series of events in individual eggs. Only by this mechanism can certain features of the process be fully appreciated and accurate data obtained on the timing of events. With the addition of time-lapse photographic methods, some of the more subtle changes become more amenable to study. Among the limitations of the technique are its limited resolution and the necessity for examining the gametes outside their normal in vivo environment.