This book is based on the lecture course taught by the author for about three decades at Charles University. The author gives a thorough and easy-to-read account of the basic principles of the standard model of electroweak interactions, describes various theories of electromagnetic and weak interactions, and explains the gauge theory of electroweak interactions. The criterion of the tree-level unitarity is used throughout the text to check the gradual steps leading to the renormalizable electroweak theory. Five appendices expound on some special techniques of the Standard Model, used in the main body of the text.The book can be read with just a preliminary knowledge of quantum field theory. In comparison with the first edition of the book published more than 20 years ago, new passages concerning the Higgs boson are added, as well as some new problems and solutions.