Analysing different theoretical approaches to joint activity and postulating its importance in other processes' development most researchers do not discuss psychological essence of a phenomenon marked as "joint activity". The authors consider three points: joint activity subject; structure and processual characteristics; criteria of joint activity efficacy. The results of such a consideration present: 1) the differentiation of terms "group activity" and "joint activity" on theoretical and operational levels; 2) non-identity of outer (socially or experimentally determined) goals of a group and inner goals, formed by joint activity subject; 3) the idea that joint activity subject cannot be set before its start - it is formed (or not formed) during joint activity formation; 4) differentiation of interaction categories in social systems and interaction as psychological process; 5) demonstration of three possible effects of joint activity - object, personal and social-psychological - and the necessity of their correspondence with inner and outer goals.