In this chapter the basic equations governing fluid flow and associated phenomena will be presented in several forms: (2') a coordinate-free form, which can be specialized to various coordinate systems, (ii) an integral form for a finite control volume, which serves as starting point for an important class of numerical methods, and (m) a differential (tensor) form in a Cartesian reference frame, which is the basis for another important approach. The basic conservation principles and laws used to derive these equations will only be briefly summarized here; more detailed derivations can be found in a number of standard texts on fluid mechanics (e.g, Bird et al., 1962; Slattery, 1972; White, I986). It is assumed that the reader is somewhat familiar with the physics of fluid flow and related phenomena, so we shall concentrate on techniques for the numerical solution of the governing equations.