At the beginning of the paper, the basic features of the family law institution of maintenance are presented: the concept, basic principles, sources, characteristics and types of maintenance. In the continuation of the paper, attention is directed towards the maintenance of adult children in the Republic of Croatia as a kind of subtype of maintenance institute. The legal nature of the institute in question is presented, as well as its special assumptions and exceptions, ways of realization, determination, modification and termination of maintenance obligations. Also, there are examples from court practice that are the most common among disputes between parents and children. The main issues that the legislator did not regulate, and which appear as disputed in everyday life, were also presented. The following part of the work is dedicated to the maintenance of adult children in the Republic of Slovenia. The child’s prerequisites necessary for acquiring the right to maintenance are presented, as well as the method of determining the amount of maintenance and the change and termination of the obligation in question. At the end, concluding thoughts were presented as a result of a comparison of the legal regulation of the institution of maintenance of adult children in the Republic of Croatia and the Republic of Slovenia with the aim of highlighting certain deficiencies in the national regulation that cause the appearance of ambiguities and complex cases before the court.