More and more transient gas-liquid operations in pipes are being successfully applied in the oil and gas industry. Pigging in two-phase pipelines, to remove liquid accumulation or for cleaning purposes, is an important transient operation. Another important operation is the injection of (-)-as 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 safe operating procedure. In pipeline-fiser system such operations cause even more severe changes in flow conditions. A two-fluid model has been developed to determine the transient behaviour of fluids during these operations. The derived one-dimensional set of equations for each flow pattern describe the flow of fluids in all regions. Semi-implicit finite difference schemes were used to solve the initial and boundary value problem for each phase of the process - gas/pig injection, gas shut-in, slug production and gas flow out of the system. An extensive experimental program has been carried out to acquire two-phase transient flow and pigging data on a 67m long, 0.0525m diameter, 9.9m high pipeline-riser system. A computer based data acquisition system has been utilised to obtain rapidly changing and detailed information of the flow behaviour during the transient tests. The model results compare well with the experimental data for characteristics such as inlet pressure, hold-up and pig velocity.