RFID tag authentication is widely viewed as a valuable tool in the fight against product counterfeiting. In this paper we describe some of the different approaches that have been proposed and we focus on what is arguably the most secure; that of dynamic tag authentication. We highlight different ways of supporting dynamic tag authentication and provide the latest implementation results. The net result is that dynamic authentication using on-tag cryptography is a reality and should be considered for deployment before other less secure options. As a sideresult we note that the on-tag overhead when supporting an asymmetric rather than a symmetric cryptographic solution can be surprisingly light.