This paper highlights the need to curb free-riding in P2P media streaming systems and discusses the mechanisms by which this could be accomplished. Free riding, whereby a peer utilizes network resources, but does not contribute services could have a huge impact on the efficacy of blue streaming systems, leading to scalability issues and service degradation. We discuss why BitTorrent-like tit-for-tat mechanisms cannot be simply tailored and used in streaming. Even though the problem of free riding is more severe in P2P media streaming than in file sharing, the deployed systems still lack incentive schemes. In this paper, we categorize, analyze, and compare a range of incentive mechanisms proposed for P2P streaming systems in the literature and discuss future research issues for these schemes to be deployed in practice.