Undergraduate computer science students and computer engineers typically have a practical orientation: they study computing because they love programming and have good performance in it. The book is mainly addressed to this core of students. The level of presentation does not require any knowledge of computer architecture, operating systems, formal languages ​​and automata theory, mathematical logic, or inductive proof techniques. The main goal of a course dealing with programming languages ​​concepts is to help the student understand programming languages ​​at an unusual level of abstraction. To achieve this goal, the book includes many exercises and examples of "real" programming, in order to encourage students whose interest in programming is primarily practical. It does not require high mathematical maturity on the part of the reader. It is designed to guide students to conceptually process the abstract concepts of programming languages, starting from simple programming exercises. The chapters of the book are divided into two categories: practical and philosophical. The first are independent "alphabets" for three programming languages ​​of different families: ML, Java, and Prolog. The philosophical chapters present the theoretical side: the fundamental principles of programming languages. These chapters alternate with practical chapters, in a sequence that facilitates the presentation of theoretical issues, so that their connection to programming practice is as visible as possible.