This paper analyzes the effects of the Erasmus + program on strengthening the mobility of secondary school teachers in Croatia with the aim of improving the knowledge, skills and competencies needed to do work in their home institutions. The main objective is to determine the extent to which the Erasmus + program has been used to strengthen teacher mobility in secondary schools during the 2014-2020 financial period. Student and staff mobility is a major activity of the Erasmus + program. The Agency for Mobility and EU Programs provides organizational support of mobile secondary school teachers who work with colleagues in partner secondary schools. Mobility of secondary school staff is a very important aspect of the internationalization of secondary schools and includes various types of international activities. Within the expected outcomes of K1 activities, high school teachers improve the quality of teaching as well as their language competencies, they participate more actively in society and increase their own opportunities for professional advancement.The research question of this paper is the extent to which the Erasmus + program has influenced the mobility and professional development of high school students in Croatia. The thesis is that the Erasmus + program significantly improves the mobility of high school students and has a positive impact on the educational, social and professional development of the individual because it improves knowledge, skills and expands horizons. It also has a positive effect on the possibility of getting a job, it helps to gain experience in international cooperation, and encourages curiosity and innovation. In the future, it is expected that the Erasmus + program will increase support for projects that encourage cooperation and exchange of practice, thus providing the main participants with the opportunity to make better use of new technologies, and to develop innovative teaching and training methods.