The concept of necessity is deeply rooted in legal traditions of many nations. A medieval Latin sentence: “necessitas legem non habet” has its equivalents in the most of modern languages. Its meaning and importance in private and public law has evolved over hundreds of years. Despite many attempts, there is still no comprehensive juridical theory describing its numerous applications. The so-called state necessity usually serves to legitimize actions of public authorities. Therefire it is an important point of reference for the theory of constitutional law. “Status necessitatis” denotes an extreme case of a state of emergency in which the necessity does not recognize any rights or it makes its own rights. In the paper the author presents the most important views on necessity from the perspective of legal theory, e.g. necessity as an exception, as a subjective right, as a special factual situation, and also as a political argument.