Peter HughesA teaching model based on Pirie-Kieren Theory was devised for the teaching of strategic thinking for the New Zealand Numeracy Project. This was used in the design of teacher material that was progressively available to most New Zealand primary teachers in the period 2001 to 2006. The ways in which teachers mis-implemented the model in their classrooms led to some significant alterations. In particular, the meaning of imaging is better defined and support for real-time formative assessment of students to inform the teaching is revised and extended.