The book presents the fundamental theories of heat exchanger and offers several examples of applications. The main objective is to give a standard procedure that makes it possible to analyze, dimension, and optimize heat exchanger analytically. The foundations of the theory and formal methods are highlighted mainly in Chapter 2 but are scattered throughout the applications. The book's core lies in applying a concept related to thermal efficiency and applications of the laws of thermodynamics, with an emphasis on the second law, through a dimensionless theoretical analysis. Helping students, engineers, and researchers who need to delve deeper into the concepts and applications of heat exchanger are dignified for someone who calls himself an educator. For this purpose, I present reproducible applications, with clear explanations of the devices' concepts and physical and geometric details and extensive open literature on the subject, which I hope will be helpful to beginners and others interested in the matter.