All the Calculus concepts and their applications are based on functions. Most students who fail or find it very difficult to pass their calculus course are proved to have a poor understanding of the concept of function. It also involves a lack of the association between a function and its graph. These facts gave rise to the production of this text exclusively dedicated to the study of functions. The text aims at helping students overcome what would surely be a serious obstacle for them to succeed in their Calculus course. The content is presented in an easy way so that students can reach the essence of the concept. To achieve the same purpose, easy examples are given and explained in detail. A set of 566 exercises are proposed for the students so that they can practice what they have learned. An answer to each proposed exercise is also provided at the end of each chapter. Without a doubt, this text provides the students with the basis for succeeding in their study of Calculus.The book starts with a discussion on relations since functions are cases of relations. The second chapter deals with the definition of a function as a relation between sets of any type, including numerical ones. This chapter provides the foundations to focus on the study of functions of a real variable in the third chapter. These are the functions established between sets of real numbers and they constitute the building blocks of calculus. This chapter covers most of the book, as it constitutes its core. Special attention is given to the construction and use of graphs. Also, the real functions mostly used in calculus applications are studied in this chapter. The fourth chapter tackles arithmetic operations with real functions and based on them the determination of domains.The explanations are given by using an easy language accessible to all students regardless of their level of knowledge. Along with the explanations, very simple examples are presented, since the objective is to help the student understand the concepts and not make them appear as complicated topics only accessible to privileged minds. However, it is important to highlight that the students should be familiar with the symbols and the language used in propositional logic as well as having a basic knowledge of set theory. The students who are not familiar with these topics should then previously review textbooks that deal with these themes. By carefully reading the entire book without omitting any section and by doing all the exercises proposed, the students will get the knowledge on functions required to guarantee their success in their calculus courses. The author is both an engineer and an economist who graduated from the Central University of Venezuela. He also earned an MSc in Development Planning from the same university. Additionally, he earned an MSc in Economics from the Queen Mary College of the University of London, and a Ph.D. in Planning Studies from the University College London of the same university. He has been a visiting researcher at the London School of Economics, the University of Ottawa, and the University of Oxford. Dr. Gallo has more than thirty years of teaching experience in mathematics and he currently works as a Math tutor at the Houston Community College. He has also published several textbooks on both Mathematics and Econometrics.