Identifying the sources of memories is an basic ability to provide accurate accounts of events. This article reviews research investigated cognitive factors which affect young children's performance of source monitoring tasks. I first briefly describe a basic theoretical framework for source monitoring and categorization of source monitoring I then review empirical evidence regarding the factors: the similarity of sources, the identity of the agent, prospective processing, the relation of source monitoring to other cognitive skills, metacognitive understanding, and the stringency of source decisions. Finally I highlight theoretical and applied implications regarding external source monitoring ability and discuss for future research.