The book is a university textbook for functional analysis. It is designed for 3-5 year students, graduate students and teachers of mathematical faculties, as well as scientists specializing in mathematics and theoretical physics. It is based on lectures that were repeatedly given by the author at the Faculty of Mechanics and Mathematics of Moscow State University, and seminars that were regularly held by him in the academic groups of this faculty. The introduced concepts and provable statements of a general nature are illustrated by a large number of examples and exercises (tasks). The reader is required to prepare in the volume of the first two courses of mathematical faculties of Russian universities.