What is the evidence that teachers who return to learn more mathematics are better teachers? Over more than a decade we have been involved in six different projects giving teachers the opportunity to reengage with mathematics. We have returned to them to ask the teachers about the long-term impact and to give us examples. The result extends existing research to provide evidence for a broad range of positive effects. Furthermore, the strong positive affective responses imply that returning to mathematics is a welcomed and practical professional development strategy. (Contains 1 table.)