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The phenomenon of twinning is a unique, biological experiment shrouded in social and scientific curiosity. More commonplace in animals than in humans, it is a fascinating example of survival of the fittest. Co-habitation of two or more fetuses within a common uterus markedly heightens the risk of disease and death for these multiples throughout their entire life cycle, but health and survival are undoubtedly influenced most by events in the perinatal period. Ultrasonography has enabled us to be conscientious observers of this experiment, has allowed us to chronicle developmental milestones in multiple pregnancy, and has guided therapeutic interventions in the management of these challenging cases. Modem high resolution imaging technology has delivered additional information regarding the epidemiology of plural pregnancies and has given us an appreciation of some of the complex, pathophysiological mechanisms which result in poorer obstetrical outcomes.