A series of studies focused on (1) the evolution of the notion of identity of objects among infants up to 15 months of age and (2) the changing rules by which this development may be understood. Six identity tasks were presented to 60 infants divided into five age groups: 5, 7, 9, 12, and 15 months. Two objects were used in all tasks. In the first three tasks, the experimenter "transformed" one object by reorienting, rotating, or removing it. In the remaining three tasks, one object was replaced by a substitute object whose features could be either similar to or different from the object replaced; the last two tasks in this group involved the disappearance of a moving object. Since subjects had to recognize objects despite spatio-temporal changes, each of the six tasks posed a specific object identity problem to the infant. Facing such problems, infants used different strategies at different ages. Combined comparative analyses of all responses revealed main trends in the development of the concept of object identity in the sensorimotor infant. Results indicated that the evolution of the identity concept between 5 and 15 months can be understood in terms of increasingly more complete criteria which allow the infant to understand that objects remain themselves despite changes they undergo. (In conclusion, theoretical implications of the findings are discussed.) (RH)