This text introduces readers to the basic principles of functional analysis and to areas of Banach space theory. In the first part, the book develops the classical theory, including weak topologies, locally convex spaces, Schauder bases, and compact operator theory. The second part covers topics in convexity and smoothness, finite representability, and variational. The text can be used in graduate courses or for independent study. It includes exercises of different levels and is aimed at young researchers in functional analysis. It can also serve as a reference book.