This chapter on transformer deals with the different types, classifications, constructional features, cooling methodology, and basic EMF equations. The assumption for developing equivalent circuit and phasor diagram from test results, determination of equivalent circuit from self-inductance and mutual inductance and vice versa, hysteresis and eddy current losses, calculation of energy and power efficiency, % regulation, load sharing in parallel-connected transformer, concept of polarity, and auto transformer have been done. Three-phase transformer connections, Scott-connected transformer and inrush current, etc., have been dealt.