Nowadays Peer-to-Peer (P2P) systems became an important part of Internet, millions of users have been attracted to use their structures and services. The popularity of Peer-to-Peer systems speed up academic research joining researchers from systems, networking and theory. The most popular P2P applications support file-sharing and content distribution, new applications are emerging in different fields, Internet telephony is an example.

This paper discusses the issues of P2P systems such as characteristics, structures, protocols, drawbacks, open problems and futures fields of development.