Set theory is the language in which the whole of mathematics is formalised. The chapter starts from propositional logic — propositions, connectives, truth tables and De Morgan’s laws — and then shows that logic and sets are the same object written in two ways. In this way sets and their operations are introduced (union, intersection, complement, difference and symmetric difference), visualised with Venn diagrams. Then come the relations between sets, with the two great families of equivalences (which partition into classes) and orders (total or partial, represented with Hasse diagrams). It closes, as an appendix, with the principle of induction, the tool for proving properties valid for all natural numbers.