One of the basic principles that the executive authority adheres to and respects is the principle of legality، which means its obedience، commitment and respect to the constitutional and legal rules in the state. Dealing with these exceptional circumstances in a way that does not violate the constitutional and legal rules، which are called the state of emergency or the exceptional circumstances represented by its executive authority، which is over the head of state the and who has been granted in some constitutions the powers to declare a state of emergency and is prohibited from this authority in the constitutions of other countries.In this research we divided the study into theory and process in the topic related to theoretical study، we knew the state، the government and the head of the state، as well as the state of emergency، and in the practical study we dealt with the study and analysis of a number of well-known political systems and constitutions of many countries according to political systems and their comparison and the role of the head of state in declaring a state of emergency in them.