A mechanistic approach to the prediction of hydrodynamic slug initiation, growth and subsequent development into continuous slug flow in pipelines is presented. The approach is based on the numerical solution of the one-dimensional transient two-fluid model equations. The advantage of this approach is that the flow field is allowed to develop naturally from any given initial conditions as part of the transient calculation; the slugs evolve automatically as a product of the computed flow development. The need for the many phenomenological models for flow regime transition, formation of slugs and their dynamics can thus be minimized. It is shown that when the two-fluid model is invoked within the confines of the conditions under which it is mathematically well-posed, it is capable of capturing the growth of instabilities in stratified flow leading to the generation of slugs. The computed rates of growth of such instabilities compare well with the values obtained from Kelvin-Helmholtz analyses. Simulations are then carried out for a large number of pipe configurations and flow conditions that lead to slug flow. These include horizontal, inclined and V-section pipes. The results of computations for slug characteristics are compared with data obtained from the literature and it is found that the agreement is remarkable given the simplicity of the one-dimensional model. (C) 2002 Elsevier Science Ltd. All rights reserved.