Objectives The research objective is to present, analyse and discuss all available regulatory guidelines, as well as the national ordinances and laws of the Republic of Croatia, which should be used while developing and approving medicinal products which contain a fixed combination of active substances. This research aims to systematize the relevant criteria for obtaining marketing authorisations for fixed combination medicinal products, with an emphasis on the conditions for approval that apply in the territory of the European Union, i.e. the Republic of Croatia. Materials and methods Research methods for the purpose of creating the subject paper include a literature search in accordance with the research topic, from general to specialized articles in published scientific papers, to scientific and expert guidelines and regulations such as directives, acts and ordinances. After a general search, articles, guidelines and acts relevant to the issue of the specialist paper were filtered, presented in more detail and critically analysed. The paper describes the structure of the European and Croatian legislative framework, as well as the consistency of legal norms in the area of approval of medicinal products containing a fixed combination of active substances. The main principles of the legal act Directive 2001/83/EC, valid on the territory of the EU, and the national Medicinal Products Act and the Ordinance on Granting Marketing Authorisations for Medicinal Products, into which the provisions of the Directive have been transferred are described. All legal documents and applicable guidelines described in the paper are focused on the regulatory aspects of the development and approval of fixed combination medicinal products. In addition, the guidelines issued by the World Health Organization and the Food and Drug Administration of the United States of America are presented, which, although not binding for the territory of the Republic of Croatia, may be useful to marketing authorisation holders for the development of the drug and the preparation of documentation for granting of marketing authorisations. Results This thesis has defined the legislative framework for the approval of medicinal products containing a fixed combination of active substances in the EU. Seven regulatory guidelines specific to the field of medicines with fixed combinations of active substances have been identified, with four issued by the European Medicines Agency and the remaining issued by the World Health Organization and the United States Food and Drug Administration. Challenges in the development and approval of such medicines have been highlighted, including the need for harmonization of existing guidelines and for the development of new guidelines, particularly in the area of fixed combinations that contain new active substances. Conclusion In the recent years, there has been an increase in the development and approval of drugs that contain a fixed combination of active substances. These types of drugs have numerous advantages, such as improved patient compliance, better drug efficacy, reduced occurrence of side effects, and lower therapy costs. When developing drugs containing fixed combinations, it is necessary to comply with all applicable regulations, as well as regulatory and scientific guidelines specific to the market for which the drug is intended. One key factor in the approval process for such drugs is providing a sufficient rationale for the use of the combination of individual active substances from a scientific and medical perspective. It is expected that the interest of pharmaceutical companies in drugs containing fixed combinations will continue to grow in the coming years, underscoring the significant role of developing new regulatory and scientific guidelines in this field.