The ability to learn and remember individuals is critical for the stability of social groups. Social recognition reflects the ability of mice to identify and remember conspecifics. Social recognition is assessed as a decrease in spontaneous investigation behaviors observed in a mouse reexposed to a familiar conspecific. Our results demonstrate that group-housed mice show social memory for a familiar juvenile when tested immediately, 30 min, 24 h, 3 days, and 7 days after a single 2-min-long interaction. Interestingly, chronic social isolation disrupts long-term, but not 30-min, social memory. Even a 24-h period of isolation disrupts long-term social memory, a result that may explain why previous investigators only observed short-term social memory in individually housed rodents. Although it has no obvious configural, relational, or spatial characteristics, here we show that social memory shares characteristics of other hippocampus-dependent memories. Ibotenic acid lesions of the hippocampus disrupt social recognition at 30 min, but not immediately after training. Furthermore, long-term, but not short-term social memory is dependent on protein synthesis and cyclic AMP responsive element binding protein (CREB) function. These results outline behavioral, systems, and molecular determinants of social recognition in mice, and they suggest that it is a powerful paradigm to investigate hippocampal learning and memory.