are required. The necessary terminology is provided at the outset, and the theory of distributions, L^p spaces, the Hardy-Littlewood maximal operator, and the singular integral operators are called upon. One of the highlights is that the proof of the Sobolev embedding theorem is extremely simple. There are two types for each function space: a homogeneous one and an inhomogeneous one. The theory of function spaces, which readers usually learn in a standard course, can be readily applied to the inhomogeneous one. However, that theory is not sufficient for a homogeneous space; it needs to be reinforced with some knowledge of the theory of distributions. This topic, however subtle, is also covered within this volume. Additionally, related function spaces-Hardy spaces, bounded mean oscillation spaces, and Hölder continuous spaces-are defined and discussed, and it is shown that they are special cases of Besov spaces and Triebel-Lizorkin spaces.