More and more transient gas-liquid operations in pipes have been successfully applied in the oil and gas industry. Pigging operation in two-phase pipelines to remove liquid accumulation or for cleaning purpose is an important transient operation. Another important operation is the injection of gas to transport the accumulated liquid in the pipeline to process facilities. Analysis of such transient two-phase flow in a pipeline is necessary not only for designing the liquid and gas handling facilities, but also for establishing safe operating procedures. In pipeline-riser systems, such operations cause even more severe change in flow conditions. A two-fluid model has been developed to determine the transient behavior of fluids during these operations. A one-dimensional set of equations for bubble/mist, annular and stratified flows has been derived. Slug flows were modeled as a combination of the foregoing. Semi-implicit finite difference schemes were used to solve the initial and boundary value problem for each phase of the pigging process: gas/pig injection, gas shut-in, slug production, and gas flow out of the system. An extensive experimental program was carried out to acquire two-phase transient flow and pigging data on a 69-m-long, 9.9-m-high, 50-mm-dia pipeline-riser system. A computer based data acquisition system was used to obtain rapidly changing and detailed information of the flow behavior during tests. The model compared well with the experimental data for characteristics such as inlet pressure, hold-up, and pig velocity. Liquid production efficiencies for different operations were compared.