The analysis so far in this book has assumed that pipelines were operating in steady-state conditions, i.e., no significant variations in time of the operational parameters such as pressures or flow rates. However, it is well-known from practice that pipelines operate under conditions or regimes that are far away from steady-state conditions. The analysis of the transient flow in pipelines is one of the most important parts of its design, but for different reasons in liquid and gas lines since the compressibility of fluids affects the flow parameters and also the mechanical design.