This book develops a systematic philosophical theory of social action and group phenomena, in the process presenting detailed analyses of such central social notions as "we-attitude" (especially "we-intention" and mutual belief, social norm, joint action, and - most important - group goal, group belief, and group action). Humans are social beings whose accounts of their social life inherently rely on social group notions involving the core concept of "we." The crucial notions for understanding macro-level group notions are shown to be joint action, we-attitude (especially we-intention and mutual belief), social norm, and group-commitment-creating "authority system" (roughly, a system for the formation of joint intentions). Though this is a philosophical work, it presents a unified conceptual framework that may be useful to social scientists, especially social psychologists, as well as philosophers.