Natural convection in an initially stagnant horizontal layer of fluid heated from below has received considerable attention since it is a simple example of convective motion generated by the instability of a basic state. Traditionally, it is called Benard convection. The principal convection mechanisms have been identified : buoyancy forces resulting from thermally induced density gradients and/or interfacial forces due to surface tension variations produced by temperature gradients. The facts and the problems of Benard convection in which turbulence gradually emerges from an ordered pattern are discussed here. Additionally, thermal convection with flowing films and convection induced by gas absorption and electric fields are introduced.