This chapter examines collective self-defence treaty arrangements. It engages with a diverse range of examples of the collective self-defence treaties (or treaties that contain collective self-defence aspects) that have emerged since 1945 to draw out common themes as to the nature, process, and role of such arrangements, as well as to establish notable variations. The aim is to contribute an overall picture of collective self-defence today specifically in the context of treaty relationships. The chapter argues that such relationships inevitably impose only weak obligations on their parties to defend each other and also can cause notable issues related to overlapping memberships, bureaucracy, and antagonism amongst members (amongst other difficulties). Equally, these arrangements – of which there are now hundreds – are concluded for good reason(s). They provide a range of benefits, especially in terms of their deterrent effect.