This chapter examines the issue of security in peer-to-peer (P2P) systems from the standpoint of trust. It takes the view that P2P systems present particular challenges in terms of trust over other socio-technical systems, and identifies three key areas of importance: identity; social contexts; punishment and deterrence. It suggests that a better understanding of these areas and the trade-offs associated with them can help in the design, implementation, and running of P2P systems. The chapter combines a discussion of problems and issues in current systems with a review of some of the wider sociological and nonsystems literature which can aid those involved with P2P systems. It concludes with some suggestions for areas where future research may provide fruitful insights.