The book is based on a course of lectures delivered to 3rd and 4th year students of the Physics Department of Moscow State University, specializing in theoretical physics. Its first part contains an exposition of the main ideas of the theory of gauge fields, the construction of gauge-invariant Lagrangians and a description of the spectra of linear excitations, including over a non-trivial ground state. The second part is devoted to the construction and interpretation of solutions of nonlinear field equations - solitons, "Euclidean bubbles," instantons and sphalerons. The third part deals with the effects arising from the interaction of fermions with topological scalar and gauge fields. The book contains the Supplement, which briefly discusses the role of instantons as saddle points of the Euclidean functional integral. The material presented can be studied in parallel with the study of quantum mechanics, and then quantum field theory. In this regard, the book is useful for both researchers and graduate students, as well as senior students specializing in the field of theoretical physics.