Peer-to-peer (P2P) data networks dominate Internet traffic. In this work, we study the problems that arise when mobile hosts participate in P2P networks. We primarily focus on the performance issues as experienced by the mobile host, but also study the impact on other fixed peers. Using BitTorrent as a key example, we identify several unique problems that arise due to the design aspects of P2P networks being incompatible with typical characteristics of wireless and mobile environments. We then present a wireless P2P (wP2P) client application that is backward compatible with existing fixed peer client applications, but when used on mobile hosts can significant improve performance.