Constraint scheduling is a dynamic process of arbitrating between competing users sharing a finite resource. Cross-layer scheduling allows vertical interactions between some protocol layers to optimize system performance. This article proposes a cross-layer scheduling for orthogonal frequency-division multiplexing (OFDM) based wireless networks. The scheduler combines opportunistic communications and link adaptation to serve users with multiple concurrent flows (applications) of different quality of service (QoS) requirements. For each time slot and on each OFDM subcarrier, the scheduler computes a cost function for each flow that depends on a flow's instantaneous link quality and service history. The flow with the least cost on an OFDM subcarrier is assigned the subcarrier, provided no other flow's delay constraint is in violation. The scheme is scalable, optimizes wireless throughput, traffic delay-aware, guarantees minimum service to all active flows, and has low implementation complexity.