Theoretical relations that can be used to predict transition from a stratified pattern to an intermittent pattern for gas-liquid flow in long pipes are presented. These include Kelvin-Helmholtz theory, viscous long wavelength theory, the necessary conditions for a slug to be stable (slug stability theory), and the definition of a plug flow. Comparisons are made with measurements of transition taken in systems over a range of pipe diameters and a range of fluid properties. An important result is that slug stability defines transition in flows with high gas density. A methodology for predicting transition is proposed. (C) 2002 Elsevier Science Ltd. All rights reserved.