This thesis is about the construction of the real number system. First by Dedekind´s cut and second by Cauchy sequences. Maybe we have an intuitive concept of the real number system. It is the number system that should be used for measurement of space and time and as well for other quantities such as mass and temperature that are thought of as varying continously rather than discretley. Of course there is not much to learn if we don´t understand the natural numbers, the integers and the rational numbers. To understand them we have to know some about sets, functions and relations. Therefore this thesis begin with a short chapter of sets, functions and relations. Then we are able to follow the developing of the number system, from naturals to rationals and finally to the reals. Most proofs are those of importance for construction and completeness of each number system.