The aim of this paper is to examine the current state of the treatment of the topic of sustainable development in the teaching of secondary schools in Croatia. The paper consists of a theoretical overview of the topic and an empirical part, i.e a display of the conducted research. The first part of the paper presents the basic theoretical principles of the concept of sustainable development, the historical overview of the origin of that concept and its presence in the educational system. Subsequently, the paper theoretically analyzes Sustainable Development as one of the so-called cross-curricular topics in the Croatian educational system and its corresponding Curriculum. The next part of the paper describes the research that was conducted via interviews with ten subject teachers of Zagreb secondary schools. The aim of the research was to empirically analyze the implementation of the topic of sustainable development in Croatian secondary schools and to compare the collected experiences with the existing guidelines of the Curriculum for the topic of Sustainable Development. The results showed that all of the interviewees are familiar with this topic and implement it in their teaching in some form. As positive methods and experiences regarding the topic of sustainable development, the interviewees singled out school projects, field teaching, cross-curricular and extracurricular cooperation, with an emphasis on the practical work of students. Some of the obstacles to the working on the topic of sustainable development that appeared among the interviewed professors are related to the reactions of students, such as demotivation and misunderstanding of the concept, as well as to being limited by the school timetable. It should be mentioned that Sustainable Development is only one of the existing seven cross-curricular topics in the Croatian school system, but this research showed that the interviewees implement the Curriculum guidelines for this cross-curricular topic in their teaching.