This chapter serves as an introduction to all the following chapters. Fluid motions manifest themselves in many different ways. Some can be described very easily, while others require a thorough understanding of physical laws. In engineering applications, it is important to describe the fluid motions as simply as can be justified. This usually depends on the required accuracy. Often, accuracies of ±10% are acceptable, although in some applications higher accuracies have to be achieved. The general equations of motion are very difficult to solve; consequently, it is the engineer’s responsibility to know which simplifying assumptions can be made. This, of course, requires experience and, more importantly, an understanding of the physics involved.