“The participation of workers in making decisions is a democratic achievement that reduces inequalities based on differences in economic strength, power, rights, privileges and property” (Herman, Ćupurdija, 2011: 124). In the Republic of Croatia, the system of co-determination of workers with the employer is based on the Constitution of the Republic of Croatia and is regulated in more detail by the legal norms of the Labour Act. According to the principle of constitutionality and hierarchy, all laws must be in accordance with the Constitution, and according to the principle of legality - other regulations must be in accordance with the Constitution and laws. Through the participation of workers in decision-making, all rights and interests of workers in economic and social terms are protected. The aim of this paper is to describe the beginnings of legal standardization of workers' rights, the implementation of the legislation in the Republic of Croatia and legislative approximation with the acquis communautaire. The work also presents and elaborates all forms of co-determination of workers with the employer based on the Labour Act and other regulations, mostly co-determination at the level of workers' council, which is regulated in the most extensive way by the Labour Act. According to the regulations of the Republic of Croatia, the interests of workers are represented by workers' councils and trade unions. Although trade unions are associations that represent the interests of their members, the practice shows that they raise the level of rights for all other workers. This leads to the identification of the two terms: workers' council and trade union. At the end of the paper, the differences and similarities between these concepts are presented.