This volume is a substantially revised new edition of the earlier book of the same title. Six new chapters (14-19) deal with topics of current interest: multi-component convection diffusion, convection in a compressible fluid, convenction with temperature dependent viscosity and thermal conductivity, penetrative convection, nonlinear stability in ocean circulation models, and numerical solution of eigenvalue problems. The book presents convection studies in a variety of fluid and porous media contexts. It begins at an elementary level and should be accessible to a wide audience of applied mathematicians, physicists, and engineers.