We study two liveness verification problems for broadcast networks, a system model of identical clients communicating via message passing. The first problem is liveness verification. It asks whether there is a computation such that one of the clients visits a final state infinitely often. The complexity of the problem has been open since 2010 when it was shown to be P-hard and solvable in EXPSPACE. We close the gap by a polynomial-time algorithm. The algorithm relies on a characterization of live computations in terms of paths in a suitable graph, combined with a fixed-point iteration to efficiently check the existence of such paths. The second problem is fair liveness verification. It asks for a computation where all participating clients visit a final state infinitely often. We adjust the algorithm to also solve fair liveness in polynomial time.Related Work We already discussed the related work on safety and liveness verification of broadcast networks. Broadcast networks [12,36,20] were introduced to verify ad hoc networks [28,35]. Ad hoc networks are reconfigurable in that the number of clients as well as their communication topology may change during the computation. If the transition relation is compatible with the topology, safety verification has been shown to be decidable [27]. Related studies do not assume compatibility but restrict the topology [26]. If the dependencies among clients are bounded [30], safety verification is decidable independent of the transition relation [38,39]. Verification tools turn these decision procedures into practice [31,15]. D'Osualdo and Ong suggested a typing discipline for the communication topology [16]. In [4], decidability and undecidability results for