This paper deals with the principles of administrative procedure under the General Administrative Procedure Act, which entered into force on 1st of January, 2010. It gives a detailed account of all the principles, content and meaning of each principle. We consider principles to be the highest norms of the legal system that express the fundamental values and to interpret all other legal norms. The importance of a correct understanding of the principles for the interpretation of procedural norms and the correct application of the law is emphasized. Administrative procedure as a procedure for resolving the rights and interests of citizens is conditioned by the proper application of these principles. Their meaning is reflected in the obligation to strictly adhere to the principles throughout the proceedings. Particular attention is paid to the principle of proportionality in protecting the rights of parties and the public interest. The development of the principle begins in the 17th century and continues until today when it is widely accepted in the legal systems of European and non-European countries as well as in the law of the European Union. The principle of proportionality is a new principle in the Croatian legal system, which was given special significance by the amendments of the Constitution of the Republic of Croatia in 2000. The introduction of the principle of proportionality as a general constitutional principle gained a confirmation of importance in the Croatian legal system. In the European Union law, the principle of proportionality is one of the most important principles, and its development has had a significant impact on the practice of the European Court of Justice. The application of the principle of proportionality when making administrative decisions is particularly pronounced in the cases in which the decision is made by a free assessment of the public law bodies. When a public law body applies an alternative legal rule in its decision making that allows one of the alternatives to be offered, it is necessary to act in its own discretion, based on a free assessment. The free assessment is the selection of the public law body between several statutory funds for the purpose of achieving a legally defined goal. The application of the principle of proportionality is prescribed in several administrative areas, but the meaning and content of this principle are not more detailed than its design is left to administrative-judicial and administrative practice.