Authentication is the process where claims of identity are verified. Most mechanisms of authentication (e.g., digital signatures and certificates) exist above the physical layer, though some (e.g., spread spectrum communications) exist at the physical layer often with an additional cost in bandwidth. We introduce a general analysis and design framework for authentication at the physical layer where the authentication information is transmitted concurrently with the data. By superimposing a carefully designed secret modulation on the waveforms, authentication is added to the signal without requiring additional bandwidth, as do spread spectrum methods. The authentication is designed to be stealthy to the uninformed user, robust to interference, and secure for identity verification. The tradeoffs between these three goals are identified and analyzed in block fading channels. We describe further extensions to OFDM and multicarrier wireless devices. Finally, both simulation results and experimental results from laboratory implementation of these new schemes are given that demonstrate the potential of this new authentication technique.