This chapter covers heat transfer by natural or free convection heat transfer driven by gravitational force (buoyancy) due to the fact that the density decreases with increasing temperature. The first situation analyzed is heat transfer by natural laminar convection over isothermal surfaces. For this, differential conservation equations are presented whose solutions are given in Appendix B.2. Dimensionless numbers of natural convection are the Grashof and Rayleigh numbers. Practical working correlations of the Nusselt number as a function of Grashof (or Rayleigh) and Prandtl are presented for several configurations and boundary conditions for both laminar and turbulent flow regimes. Practical working correlations are presented for natural convection confined between two or more vertical, inclined, and horizontal surfaces as well as for a confined space. Various solved problems are presented. At a low fluid velocity the phenomenon of simultaneous natural and forced convection may be relevant and for this purpose criteria for mixed convection are established.