Pressure drops represent one of the most studied and relevant themes of thermohydraulics. In all fluid systems, pressure drops are responsible for the required pumping power; in parallel channels, they influence the flow distribution among the channels; in two-phase flows, an accurate evaluation of pressure drops is particularly important for the possibility of unstable conditions. The treatment of single-phase flow pressure drops refers both to liquid and gas or vapor flows. Outside tubes, the flow is less confined and its local characteristics are less repeatable. The limits of the reproducibility of the flow and the difficulty of individuating a few parameters sufficiently representative of the local flow conditions, already pointed out for parallel flows outside tubes, increase considerably in cross flows. The homogeneous model is applicable satisfactorily to flows with extreme void fractions, high pressure, and high mass velocities. The effect of local boiling is particularly important in pressurized water nuclear reactors.