In this chapter, the mathematical formulation of the convective heat transfer problem is presented. The governing equations for natural convection in two-dimensional fluid and saturated porous layers are described first. Thereafter the boundary conditions for the problem are elucidated. In particular, boundary conditions at the interface of the fluid and porous layers are discussed in detail. The one-domain formulation is then derived, and the governing equations are presented in dimensionless form using the vorticity-stream function formulation.