Recent progress in modeling expiratory flow is reviewed. Two idealized flow limitating mechanisms are described: flow limitation at wave speed and viscous flow limitation. In the lung, both convection acceleration and viscous dissipation contribute to the pressure distribution that determines airways compression, but the first dominates at high and mid lung volumes and the second dominates at low lung volumes. A computer model describes the combined effects. This quantitative modeling is consistent with long-standing qualitative concepts of flow limitation and provides a basis for future work on modeling expiratory flow limitation in abnormal lungs.