Power transformers have been introduced since the early days of power transmission. Its development combined with the development of circuit interrupting devices, interrupters, and circuit breakers has contributed to the establishment of ac power transmission as the principal mode of transmitting electric power over long distances as well as distributing it to the consumer. However, while the demand for higher interrupting capacity and the high risk of fire hazards have favored the diversification of circuit interrupting devices from bulk oil breakers into new technologies using nonflammable air and SF6 as the arc quenching medium, the basic design of power transformers did not change significantly during the last hundred years while progressing to meet the ever-increasing needs of modern power transmission system. It reached the voltage level of 735 kV in the 1960s and then crossed the 1000 kV level in the 1990s. Its power rating has progressed from an initial few kVA to reach several hundreds of MVA in today’s transformer units. Such a performance of power transformers is the result of the continuous improvement in material application and technology.