Cooperation threatens to become rationally problematic insofar as the following conditions hold: reliance has a worst outcome—we rely and the other proves unreliable; the interaction is one-off; and we are ignorant of the other’s particular motivations but recognize a general motivation to be unreliable. The problem is that the satisfaction of these conditions is commonplace. Thus cooperation should be much less common than it in fact is. So what explains it? This chapter considers and rejects various game-theoretical solutions before canvassing (...) a ‘trust-based’ solution. According to this solution the problem is dissolved once one recognizes how trust itself can give reasons for cooperating. (shrink)