The book addresses the relationship between criminal law and fundamental rights from the perspective of the principle of proportionality. It should be emphasised, however, that the focus given to this principle is on one of its ’faces’, namely the principle of the prohibition of insufficient protection. It also deals with what is known as guarantorism, addressing not only its negative side, which is better known and sometimes seen as the only one, but also its positive side, which is not so widespread, but whose application is gaining strength in Brazilian doctrine and jurisprudence.