The administrative law of the Republic of Croatia includes a complex system of legal norms and acts that regulate the organization, powers and procedures of public administration and relations between state bodies and citizens. Through the analysis of the legal sources of administrative law, we investigated the diversity and complexity of the sources of law that shape the operation of the administrative system and ensure the protection of citizens' rights. In the second part of the paper, the historical development of administration and administrative law is analyzed. Then, in the third part of the paper, we investigated the legal sources of administrative law, including the Constitution, laws, sub-legal general normative acts, international treaties, the practice of state bodies and the rules of science, and their role, application and influence on administrative law. In the fourth part of the paper, sub-legal general normative acts are analyzed, which represent an important part of the legal system of the Republic of Croatia. Special attention is paid to sub-legal general normative acts under special authority, acts for the execution of laws, acts that supplement and replace laws and spontaneous by-law general normative acts.