What has been illustrated so far about the finite volume method (see Chap. 3) assumed the velocity field as known. Some of the techniques for determining this field will be illustrated in this chapter. In this regard, it is useful to highlight that the link—the coupling—existing between pressure and velocity, together with the non-linearities resulting from the presence of advective terms, represents one of the major difficulties in solving the conservation equations of mass and momentum. As seen previously, the total number of equations to be solved depends both on the number of cells in the computational domain and on the conservation equations considered: for example, in the case of two-dimensional laminar compressible flow, it will be necessary to consider the conservation equation of mass, the two components of momentum conservation, energy conservation, and the state equation—five equations for each cell.