The legal system is seen as a means of regulating social behavior; to fulfil this function, its essential quality must be stability, so that its reactions to what is happening in society —both in relations between individuals and between the state and the governed—are predictable. To this end, there is a principle, that of the legal security, by virtue of which legal subject must be able to rely on the stability and predictability of the law. But it is not always possible; the configuration of the factual situations require often that a space of liberty in the application of the law has to be given to the authority, creating uncertainty on the solution it will be chosen finally. The text considers the problems they are set thus on sight of the principle of the predictability.