Fluid mechanics is the most important unit operation in chemical industries. Nearly 30% of the total plant cost is from pipes and fittings. Proper knowledge is required for correct sizing of pipes, which in turn depends upon the proper estimation of pressure drop. For liquids and compressible fluids, system pressure drop can be calculated quite accurately; however, for two-phase flow, the calculation is extremely complex. The accuracy of a two-phase pressure drop calculation largely depends on the accuracy of liquid holdup calculations. Large numbers of correlations are available for liquid holdup and two-phase pressure drop calculations. It is often confusing for the designer to use the proper equation. Attempts will be made to establish general guideline for the use of two-phase correlations.