This longitudinal quantitative study investigated teachers’ development of self-efficacy andteaching 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 locally discuss and experiment with the principles, complexities and challenges of ESD. Data was collected at five different time points using a questionnaire including scales measuring teachers’ self-efficacy for ESD and their self-reported ESD practices. Results show that the teachers’ self-efficacy was boosted early in the program, but it fell back to initial low levels after confrontation with practice. Through further experimentation in their own practices, the teachers’ self-efficacy for ESD increased back to the initial level toward the end of the program. Furthermore, teachers started self-reporting actual implementation of ESD practices as the program progressed, and the correlation between teachers’ self-efficacy for ESD and self-reported 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.