Book excerpt: This professional learning programme for Key Stage 3 mathematics teaching is grounded in the latest research on the characteristics of effective professional development. The materials help teachers: - deepen their content knowledge for important mathematical concepts in their grade - increase their understanding of how students learn these mathematical ideas - use their knowledge to develop effective lessons and improve instruction - enhance their collaboration skills. The mathematical content of Teaching by Design in Mathematics matches content topics in number and operations identified for each grade by the NCTM Curriculum Focal Points. The culminating activity of the programme is the co-creation of a prototype lesson which is taught to students by team members; the team then investigates the impact of the lesson on student learning. The cycle of investigating, planning, teaching, observing, debriefing, and revising a lesson together contributes to a climate of continuous professional learning.