Currently, the Ombudsman is a traditional component of democratic legal systems. The creation of such a body, as noted in the Council of Europe Resolution «On the role of commissioners/ombudsmen in the protection of citizens’ rights», which would try to ensure justice, respect for the foundations of the rule of law and at the same time be able to establish a dialogue with citizens, is necessary in many States. The purpose and objectives of this article are to consider the issues of the emergence and development of the constitutional-legal institution as an Ombudsman in General, and the evolution of this institution, in which there were various models and types of ombudsmen. It is also necessary to describe the existing models of the Ombudsman applied in different States. The article examines the functional specialization of ombudsmen, which occurs through the introduction of ombudsmen in certain areas of public relations or to protect the rights and interests of the most legally vulnerable categories and groups of the population, and specifically the emergence of the institution of migration ombudsmen. The methodology of the article is based on a set of philosophical and worldview, General scientific principles and approaches and special scientific methods of cognition of constitutional and legal phenomena. When writing, a number of General scientific and special scientific methods were used, including: system and structural-functional methods, sociological method, formal-logical method, comparative-legal method. As a result of the research, we can conclude that the essential characteristics of the Ombudsman institution have changed from the institution of supervision of the administration and the court to the most important human rights mechanism that it represents at the present stage. Within the framework of the functioning of the institution of the Ombudsman, different models have been identified in different States and specialized ombudsmen have appeared, including those dealing with the protection of the legal rights and interests of migrant workers.