Providing streaming service over the Internet is a challenging problem. The difficulty is twofold. First, it is not a trivial task to stream media on an end-to-end basis because of a video's high bandwidth requirement and long duration. Second, scalability issues arise when attempting to service a large number of clients. Traditional streaming service employs a client-server unicast service model. As the media popularity increases, the server soon becomes the bottleneck. The problem of how to best provide media streaming service to a large number of clients in a scalable manner remains to be solved. The recently emerging P2P technologies have huge potential on resource usage and system scalability, and P2P-based media streaming is attracting a great deal of attention. Research advances in key technology of P2P-based media streaming are discussed and the state of art of P2P media streaming is presented. Finally, some issues to be further studied are discussed.