This article provides an overview of the current state of knowledge regarding heat transfer technology in high and very high power transformers, focusing on transformer cooling, heat dissipation without insulation deterioration, and the calculation of temperatures for the hottest points of the winding. It also discusses the rotation of the heat flux field, which induces additional forces that alter the heat transfer pattern of the wall channel, causing secondary radial fluxes with high temperature gradients. Erosion of ceramic and metallic materials on the lamination surfaces is outlined. Reference is made to the cooling of the surfaces of the cooling fluid tank and cooling tubes, as well as the hottest points and temperature transients. Attention is given to heat dissipation in the cooling fluid medium. Finally, calculations of the transient response are made without considering the delay due to the transport of the insulating fluid, and the influence of this in a very high power transformer is noted.