For more than a century the wake flow behind a cylinder has been the subject of many investigations. Most attention has been focussed on the first instability and the transition towards 3D for the flow around an unheated cylinder. The wake stability for a heated cylinder has until now received very little attention compared to the forced convection case. After a review of the literature on the wake flow behind an unheated and heated cylinder, in this paper the 2D wake behaviour and the 3D flow transition behind a heated cylinder are described. In the analysis performed, the Reynolds number is set around ReD=100 and the Richardson number is varied between RiD=0 (forced convection case) and RiD=1.5 (mixed convection case). From the results it is seen that for a relatively small heat input (RiD1) an early 3D transition is observed. Mushroom-type structures appear on top of the upper vortex row. This flow transition is initiated by the occurrence of 3D flow structures at the rear end of the cylinder.