This longitudinal quantitative study investigated teachers' development of self-efficacy and teaching practices relating to education for sustainable development (ESD) in four compulsory schools in a Swedish municipality. The teachers participated in a professional development program over three school years designed to support them in implementing ESD. The program was based on five seminars that supported teachers to discuss and experiment with the principles, complexities and challenges of ESD. Data was collected at five different time points strategically planned at key moments in the program, using a questionnaire including scales measuring teachers' self-efficacy for ESD and their self-reported ESD practices. Descriptive and inferential statistics were used to follow the teachers evolution across a time span of about three years. Results show that the teachers' self-efficacy was boosted early in the program, but fell back to initial levels after confrontation with practice. Through further experimentation in practice, the teachers' self-efficacy increased back to the initial level toward the end of the program. Furthermore, teachers started self-reporting ESD practices as the program progressed, and the correlation between self-efficacy for ESD and ESD practices grew. These results highlight the importance of providing teachers with long-term opportunities for bringing ESD into their own educational practice. The results also caution against using self-efficacy as an outcome measure in short-term professionalization initiatives.