In wireless networks, users share a transmission medium. To increase the eficiency of channel usage, wireless systems often use a Medium Access Control (MAC) protocol to per- form channel coordination by having each node announce its usage intentions; other nodes avoid making conicting trans- missions minimizing interference both to the node that has announced its intentions and to a node that cooperates by avoiding transmissions during the reserved slot. Tradition- ally, in a multi-channel environment, such announcements are made on a common control channel. However, this con- trol channel is vulnerable to jamming because its location is pre-assigned and known to attackers. Furthermore, the an- nouncements themselves provide information useful for jam- ming. In this paper, we focus on a situation where multiple wireless transmitters share spectrum in the presence of intel- ligent and possibly insider jammers capable of dynamically and adaptively changing their jamming patterns. We develop a framework for efiectively countering MAC- aware jamming attacks and then propose SimpleMAC, a protocol resilient to these attacks. SimpleMAC consists of two schemes (the Simple Transmitter Strategy and the Sim- ple Signaling Scheme) that are easily analyzed using game theory, and show the optimal adversarial behavior under these protocols. We evaluate our schemes mathematically, through Monte Carlo simulations, and by implementation on the WARP software-defined radio platform. SimpleMAC provides very rapid improvement over the alternative of not using any MAC protocol, and eventually converges to opti- mal performance (over six-fold improvement in SINR, 50% gains in Shannon capacity in a realistic mobile scenario).