This article presents the results of a continuing professional development (CPD) study of teachers in Education for Sustainable Development (ESD) in Croatia. This article and research aimed to determine the contribution of educational forms to the likelihood of teachers becoming involved in CPD for ESD, as well as to engage in a discussion about the Competence-based approach in ESD. The tasks of the research were to determine the level of achieving ESD competences, the correlations with educational forms/training, and the contribution of forms of education/training for CPD for teachers in ESD. A survey questionnaire was developed, which contained a scale of self-assessment of teacher competencies for ESD. The sample of respondents comprised 1096 primary school teachers. The results indicate that an average high level of achievement competences was obtained, as well as a relation to individual forms of education. The results indicate 28% of the explanation of the total variance with regards to the contribution of educational forms to the level of achieved teacher competencies for ESD and a very small contribution to the likelihood of inclusion into CPD for ESD. The research points to the inadequate implementation of ESD in the CPD of teachers.