An introductory course on analog and digital communications is fundamental to the undergraduate program in electrical engineering. This course is usually offered at the junior level. Typically, it is assumed that the student has a background in calculus, electronics, signals and systems, and possibly probability theory. Bearing in mind the introductory nature of this course, a textbook recommended for the course must be easy to read, accurate, and contain an abundance of insightful examples, problems, and computer experiments. These objectives of the book are needed to expedite learning the fundamentals of communication systems at an introductory level and in an effective manner. This book has been written with all of these objectives in mind. Given the mathematical nature of communication theory, it is rather easy for the reader to lose sight of the practical side of communication systems. Throughout the book, we have made a special effort not to fall into this trap. We have done this by moving through the treatment of the subject in an orderly manner, always trying to keep the mathematical treatment at an easy-to-grasp level and also pointing out practical relevance of the theory wherever it is appropriate to do so.