This textbook presents an introduction to the fundamental concepts of continuous-time (CT) and discrete-time (DT) signals and systems, treating them separately in a pedagogical and self-contained manner. Emphasis is on the basic signal processing principles, with underlying concepts illustrated using practical examples from signal processing, multimedia communications, and bioinformatics. Following introductory chapters, the text is separated into two parts. Part I covers the theories, techniques, and applications of CT signals and systems and Part II discusses these topics for DT, so that the two can be taught independently or together. With over 300 illustrations, 285 worked examples and 385 homework problems, this textbook is an ideal introduction to the subject for undergraduates in electrical and computer engineering.