One-dimensional models are presented for simple, separated (stratified, annular) and mixed (bubble, droplet) gas-liquid flows. Specific attention is focused on equations for (i) interface and wall transfers of heat and momentum; (ii) the difference between phase and interface pressure; (iii) interface pressure; and (iv) space-time distribution parameters. Equations appropriate to simple separated and mixed flows are investigated to determine their effect on the propagation velocities of acoustic and interfacial waves. Solutions to two simple numerical problems, obtained by using these equations, are also discussed. Finally, recommendations are made for a more general transient model.