This tutorial provides the basis for an understanding of fundamental concepts and application issues in the design of high frequency magnetic circuits and transformers. It covers the basic theory, including BH curves, transformer models, magnetic design equations, simplifying assumptions and rules of thumb. Practical issues are addressed such as core materials and selection, conductors and insulation and construction issues. High frequency design issues are discussed: conductor skin and proximity effects, core losses considering dc and harmonic components. Application examples are given and design procedures for high frequency transformers and flyback transformers are illustrated by means of spreadsheet approach.