Under Swiss law so-called contracts for prostitution are considered void as they are contra bonos mores or «immoral» according to article 20 paragraph 1 of the Code of Obligations. Yet what is considered to offend against common decency is unclear, and the definition of the term «immoral» can change over time. The authors outline how this change has affected court rulings and academia in Switzerland, Germany and Austria. They argue that contracts for prostitution are not contra bonos mores (any more). Such contracts are primarily governed by the provisions on the mandate; it is not necessary to introduce special legislation.