Departments are typical of Flemish secondary education, which is organised in subjects. Departments are much more than the administrative units into which secondary schools are divided. They are important units for organising education and learning in secondary schools. In secondary schools, a ‘department’ usually refers to a group of teachers who teach the same subject within the school. Recent research shows that departments have the potential to function as professional learning communities (PLCs). Nevertheless, research shows that the importance of departments as PLCs and how they can be facilitated in secondary schools has not yet been fully addressed. A PLC refers to a group of people sharing and critically interrogating their practice in an ongoing, reflective, collaborative, inclusive, learning-oriented, growth-promoting way. Three characteristics are put forward in the international literature: (1) collective responsibility, or the extent to which teachers feel collectively responsible for the learning of all students in the school; (2) reflective dialogue, or the extent to which teachers discuss educational issues with each other such as the curriculum and evaluation; and (3) deprivatized practice, or the extent to which teachers open up their teaching practice/classroom to each other, so that they can observe each other and provide feedback. Thus, departments as PLCs have the potential for student teachers to acquire teacher competences, to interact with experienced teachers and to learn from experienced teachers. Recent research also shows that departments as PLCs can also realise school development through didactic research. The project initially focuses on departments of bottleneck subjects.