This chapter is devoted to highlighting the main active and common materials used in fabricating power transformers as well as their most important characteristics. More specifically, the chapter focusses on the main characteristics of conductors used in windings, magnetic materials used in cores, and insulators. While there are different available options for each of the aforementioned materials that vary in quality and price, the choice of the most appropriate material is usually based upon the design and market limitations. This fact will be discussed and clarified through the content of this as well as subsequent chapters.