A professional development series designed to familiarize middle school mathematics teachers with four concepts identified by TIMSS (the Third International Mathematics and Science Study) as crucial to students' future success: proportionality and similar figures, patterns and functions, polygons and angles, and sampling and probability. In the Discovery segments, master teacher Jan Robinson leads a group of classroom teachers in investigating a series of problem-based activities. The teachers customize and expand upon these lessons in their own classrooms, then return with samples of their students' work. In the In Practice sessions, they report on their experiences, evaluate the work, and develop new instructional and assessment techniques.