The role of isovolume pressure flow curves in directing attention to expiratory flow limitation and in the development of the flow volume curve is reviewed. The approaches to modelling the flow-limiting mechanism are traced from the equal pressure point concept to current concepts that suggest that there are two basic mechanisms involved. One is the wave-speed mechanism resulting from the coupling between airway compliance and the pressure drop due to convective acceleration. The other is the coupling between airway compliance and viscous losses in the flow. A computational model for a uniformly emptying lung is presented. The model predicts the pressure distribution in the airways, isovolume pressure flow curves, and flow volume curves. The model tested well against data obtained from excised human lungs. Potential limitations of this model are discussed, as are areas requiring further development.