The aim of this research was to classify the opinions of principals and teachers working in High Schools on the Erasmus+ Key Action 1 Project regarding their reasons for joining this Project, the preparations made before joining the Project, and the benefits derived from participating in the Project. The research is a qualitative study with a multiple holistic case study design. Data were collected from interviews using semi-structured interview forms. At the end of the research, conclusions were classified under three themes. The first theme was that the main reason for joining the project was professional development. The second theme was that the preparations made by high school principals and teachers before participating in the Project consisted of language education, research on the subject of training, as well as collecting information about transport and the host country. The final theme indicated that the benefit of the Project was the opportunity to learn the use of technology in lessons.