A comprehensive mechanistic model has been developed for gas-liquid two-phase flow in horizontal and near horizontal pipelines. The model is able first to detect the existing flow pattern, and then to predict the flow characteristics, primarily liquid holdup and pressure drop, for the stratified, intermittent, annular, or dispersed bubble flow patterns.A pipeline data bank has been established. The data bank includes large diameter field data culled from the A. G. A. database, and laboratory data published in the literature. Data include both black oil and compositional fluid systems. The comprehensive mechanistic model has been evaluated against the data bank and also compared with the performance of some of the most commonly used correlations for two-phase flow in pipelines. The evaluation, based on the comparison between the predicted and the measured pressure drops, demonstrated that the overall performance of the proposed model is better than that of any of the correlations, with the least absolute average percent error and the least standard deviation.systems. The traditional approach to solve the problem has been to conduct experiments and develop empirical correlations.Although these correlations have contributed significantly to the design of two-phase flow systems, they did not take into consideration the physical phenomena.Since the mid 1970's, significant progress has been made in this area. Models have been developed to predict flow patterns. Separate Models have also been proposed for the prediction of the flow characteristics for each flow pattern, namely stratified flow, intermittent flow, annular flow and dispersed bubble flow. However, up to date, no study has been carried out to verify the consistency and the applicability of these models.The purpose of this study is to develop a comprehensive mechanistic model for two-phase flow in pipelines by combining the most recent developments in this area. The model is then evaluated against a field and laboratory measurement data bank, and compared with several commonly used empirical correlations.