The attached paper deals with the topic of health insurance in the Republic of Croatia with an emphasis on the principle of functioning of supplementary health insurance. The aim of the paper was to provide a simple and concise overview of the concept of providing health care services in this area and to analyze and compare three different insurance service providers on the supplementary health insurance market. The comparison was made on the basis of the Croatian Institute for Health Insurance, Croatia Insurance d.d. and Uniqa Insurance d.d. The paper presents a brief historical overview of the development of insurance and explains the very concept of the term insurance and its elementary features, in order to facilitate an easier understanding of the mentioned topic. A concise overview of the division of health insurance into mandatory and voluntary health insurance is provided, which is further divided into supplementary insurance, additional insurance and private insurance, although the initial focus of the work is on the analysis of the supplementary health insurance system. That is, in the way it functions in the insurance market and which legal acts define the area of its activity.