This chapter explores intelligence sharing.1 Intelligence sharing occurs when a state (commonly termed the sender) communicates intelligence it has developed with another state (the recipient). It devotes little attention to related phenomena, including operational cooperation among intelligence agencies of different countries. After defining intelligence sharing and discussing the range of forms that this practice can take, the chapter lays out the benefits that sharing states hope to secure, and the costs and risks they run when they engage in this form of cooperation. I then describe theoretical attempts to understand the conditions under which states share intelligence, focusing on explanations that emphasise mutual trust, institutions and monitoring, and hierarchy. The chapter next discusses how researchers have in practice analysed the conditions facilitating intelligence sharing, focusing particular attention on the work of historians, policy analysts and political scientists specialising in international relations. The chapter closes with a brief suggestion: while ethical issues characterise discussions of most other areas of intelligence, they have played too small a role in developing our understanding of intelligence sharing, and future work could focus fruitfully on this area.