The book presents basic theories of transformer operation, design principles and methods used in power transformer designing work, and also includes limitation criteria, effective utilization of material and calculation examples to enhance reader’s techniques of transformer design and testing. It includes Core and winding commonly used, and their performances. Insulation structures and materials, methods for improvements on dielectric strengths on partial discharge, breakdown and electrical creepage. Losses and impedance calculations, major influential factors and methods to minimize load loss. Cooling design, and the method to obtain effective cooling. Short circuit forces calculations, the ways to reduced the short circuit forces and measures to raise withstand abilities. No-load and load-sound levels, the influential factors and trends, and abatement techniques. Auto transformer special features, its stabilizing winding function and its adequate size is discussed particularly. Tests and diagnosis. The ways to optimize design is also discussed throughout the book as a goal to achieve best performances on economic design. The book is a great reference material for engineers, students, teachers, researchers and anyone involved in the field associated with power transformer design, manufacture, testing, application and service maintenance. It gives high level of details to help future research and development to maintain electrical power as reliable and economical energy resource.