Thermal stability, operational reliability, and service life extension are dependent on effective cooling in the design of the transformer. Broadly, this chapter discusses different transformer cooling methods under the air-based and the oil-based cooling systems. The suitable methods for dry-type transformers and moderate heat dissipation are air natural (AN), air forced air natural (AFAN), and air blast (AB). For high-power transformers oil immersed cooling systems are offered by virtue of oil natural air natural (ONAN), oil natural air forced (ONAF) and oil natural water forced (ONWF), depending on the combination of natural convection and cooling media.