The popularity and wide spread usage of peer-to-peer (P2P) systems has soared over the past several years. Throughout the evolution of P2P systems the definition of P2P has changed along with the software architecture of the various P2P applications. While the initial popular usage of P2P systems was for file sharing (more specifically the sharing of music files in mp3 format) [4] the problem domain that P2P systems address today cover the range from data sharing to collaboration to distributed computing and beyond. For the continued increased usage of P2P systems, the need for security and trust arises. This chapter covers evolution of P2P systems through the examination of Napster, Gnutella, KaZaa, and BitTorrent [9], system capabilities and shortcomings, and security needs, which highlights the need for trust in P2P systems. With this basis we then present our vision for trust and security followed by a literature review of trust in P2P systems. We then introduce and develop a Universal Trust Set as a foundation for building trustworthy environment, and then our approach for implementing the set, and the future of P2P systems where we will discuss other open issues that need addressing. We close this chapter with some concluding remarks.