Much research has been conducted on exploring teacher learning and constituting Continuous Professional Development (CPD) designs for teachers. Yet, little is known about appropriate design principles of CPD for teacher trainers/multipliers who in turn are supposed to provide CPD for teachers. The German Center for Mathematics Teacher Education (DZLM) offers special CPD courses for multipliers that focus on design principles identified as relevant for effective CPD of teachers in the research literature. The present study elaborates whether these principles can be transferred to CPD of multipliers and how in turn these principles are affected by them: Do they consider themselves more competent after a CPD course that followed the design principles of teacher CPD? What relevance do they assign to these CPD design principles derived from the literature and has the course design become relevant for their own work as CPD providers to teachers? Participants were 12 multipliers immersed in a 1-year CPD course. Data was gathered via questionnaires and interviews. Quantitative and qualitative data analyses were carried out. Our results reveal a significant improvement of self-estimated knowledge and skills in mathematics, mathematics pedagogy, CPD management, and technical skills between before and 6 or 10 months after the course. The multipliers' descriptions of effective CPD focused on the design principles "competence-orientation" and "participant-orientation." In sum, all design principles were rated as highly important for their own work as CPD providers although they were modified. Thus, the present paper reveals deeper insights into the developmental processes multipliers undergo when educated according to crucial design principles of effective CPD.