This study presents initial design principles for a programme of professional learning for teachers focussed on supporting their student’s transition from primary to post-primary mathematics. The three key principles proposed to support the design of this programme are: (1) Facilitating primary and post-primary teachers to collaborate as part of a professional learning community; (2) Providing opportunities for teachers to inquire into their own practice through Practitioner Inquiry; (3) Supporting teachers in co-designing rich mathematical tasks that can support and develop students’ mathematical achievement.