This study seeks to describe the subject matter knowledge needed for teaching statistical association at the secondary level. Taking a practice-based qualitative approach, three experienced teachers were observed as they taught statistical association and interviewed immediately following each observation. Records of practice were assembled to create a compilation document to recreate each of the fifty observed class sessions along with related materials including textbook pages and student work. Analysis of the compilation documents focused on the demands upon teachers' subject matter knowledge involved in the practice of teaching. Findings regarding the knowledge required for teaching correlation coefficient are highlighted, including its computation, interpretation, sensitivity, estimation, and related terminology. (Contains 3 tables and 1 figure.)