This textbook is an easily understandable introduction to important fundamental concepts of mathematics and is aimed at both future and currently teaching mathematics teachers. The central concepts of set, relation, and function are essential components of mathematics education in all types of schools. Many mathematical relationships can be clearly and simply represented using the concept of sets. The most important foundational elements required for this are provided in the chapter "Sets." The focus of the chapter "Relations" is on equivalence relations and order relations, which can be used to structure sets. In the chapter "Functions," fundamental concepts are primarily addressed that play a role in mathematics education prior to calculus, as well as those that prepare students for differential and integral calculus.