Observations suggest that 12-month-old children show little evidence of possession rules, while 18- and 24-month-olds are still coming to differentiate purely personal possession rules from shared possession rules that take into account the other child's status or rights as a possessor. Children 12 months old exhibited the highest frequency of attempts to take one another's toys and the lowest frequency of positive object contacts such as sharing, joint play, and cooperation. For 24-month-olds, the pattern was reversed; children 18 months old fell intermediate between the youngest and oldest children in the sample. Oldest children more frequently mediated their possession exchanges with language, particularly with self-possession related language. Children 12 months of age were equally likely to resist attempts to take their toy, regardless of how long they themselves had been in possession of it or whether the taker had previously played with the toy. Children 18 and 24 months old were more likely to resist an attempt by another to take possession of a toy when the taker had not previously played with it, and least likely to resist if the taker had played with the toy extensively during the previous 30 minutes. The 24-month-olds also resisted more intensely if the taker had not had prior possession, and they additionally differentiated between whether the taker had played with the toy only briefly or for an extended period. (RH)