This textbook, Functional Analysis, based on the lecture notes of distinguished authors, is designed to cater to the needs of students who are yet to be exposed to the subject, as well as senior undergraduate- and graduate-level students at universities the world over. This textbook begins with a preliminary chapter that establishes uniform notations and covers background material in real analysis, linear algebra, and metric spaces. It is followed by chapters on Normed and Banach Spaces, Bounded Linear Operators and Bounded Linear Functionals. This textbook also deals with the concept and specific geometry of Hilbert Spaces, Functional and Operators on Hilbert Spaces, and Introduction to Spectral Theory. The appendix provides an introduction to Schauder Bases. The text being published in third edition incorporates many mathematical developments and revision of the authors lecture notes taken place since its publication in1995. This provides further clarity to the students and makes it an ideal textbook for easy comprehension of the subject. The clear explanations, numerous examples, problems, and illustrative figures also make the textbook invaluable for self-study and as a reference book.