We consider the problem of broadcasting a message from a sender to n ≥ 1 receivers in a time-slotted, single-hop, wireless network with a single communication channel. Sending and listening dominate the energy usage of small wireless devices and this is abstracted as a unit cost per time slot. A jamming adversary exists who can disrupt the channel at unit cost per time slot, and aims to prevent the transmission of the message. Let T be the number of slots jammed by the adversary. Our goal is to design algorithms whose cost is resource-competitive, that is, whose per-device cost is a function, preferably o(T ), of the adversary's cost. Devices must work with limited knowledge. The values n, T , and the adversary's jamming strategy are unknown.For 1-to-1 communication, we provide an algorithm with an expected cost of O( T ln(1/ ) + ln(1/ )), which succeeds with probability at least 1 − for any tunable parameter > 0. For 1-to-n broadcast, we provide a very different algorithm that succeeds with high probability and yields an expected cost per device of O( T /n log 4 T + log 6 n). Therefore, the bigger the system, the better advantage achieved over the adversary! We complement our upper bounds with tight or nearly tight lower bounds. We prove that any 1-to-1 communication algorithm with *