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Interference constitutes a major challenge to availability for communication networks operating over a shared medium. This paper proposes the medium access (MAC) protocol AntiJam, which achieves a high and fair throughput even in harsh environments. Our protocol mitigates internal interference, requiring no knowledge about the number of participants in the network. It is also robust to intentional and unintentional external interference, e.g., due to coexisting networks or jammers. We model external interference using a powerful reactive adversary that can jam a (1-ε) -portion of the time-steps, where 0 <; ε ≤ 1 is an arbitrary constant. The adversary uses carrier sensing to make informed decisions on when it is most harmful to disrupt communications. Moreover, we allow the adversary to be adaptive and to have complete knowledge of the entire protocol history. AntiJam makes efficient use of the nonjammed time periods and achieves, if ε is constant, a Θ(1)-competitive throughput. In addition, AntiJam features a low convergence time and has excellent fairness properties, such that channel access probabilities do not differ among nodes by more than a small constant factor.