Since there are two known bases of inheritance in the Croatian inheritance system, the law and the testament, an overview of both bases, with emphasis on the testament, will be given at the beginning of this thesis. It will overview the notions of testamentary capacity, the contents of the testament, and the reasons which could make the testament invalid. At the core of this thesis is one of the reasons that make a testament invalid, or because of which the testament can be contested, that is the disposal of a testament contrary to someone’s forced inheritance right. The right to contest the testament belongs to the heirs in case the testator violated their subjective rights to a vital part of the testament. Testamentary regulations that infringe one’s rights to a compulsory portion are contestable, so the heirs who are entitled to it have the right to contest them. The thesis will describe the concepts of forced inheritance, compulsory portion, and potential heirs entitled to a compulsory portion, as well as the institutes of rightful inheritance of forced heirs and the rights that belong to them in case their obligatory portion is violated. The theoretical part will be accompanied by examples from case law.