Assessment for Learning (AfL) is complex requiring teacher competences for collecting, analysing and interpreting evidence of student progress, and for adapting instruction and learning accordingly. In this study, the four-component instructional design (4C/ID) model for complex learning was used to design a teacher professional development (TPD) programme for developing teachers' AfL competences. The evaluation of the implementation of the program showed that teachers were positive about the relevance and usability of the program, and about the AfL knowledge, skills and attitudes gained. This study yields important lessons about how to support teachers in learning to apply AfL in their classrooms. The study shows that the 4C/ID model can provide the learning-psychological basis for the design of TPD programmes for the acquisition of complex teacher skills, that require the integration of skills, knowledge and attitudes.