Learning Mathematics for Teaching (LMT) project investigates the mathematical knowledge needed for teaching, and how such knowledge develops as a result of experience and professional learning. We do so through the writing, piloting, and analysis of problems that reflect real mathematics tasks teachers face in classrooms - for instance, assessing student work, representing numbers and operations, and explaining common mathematical rules or procedures. Assessments composed of these problems are often used to measure the effectiveness of professional development intended to improve teachers' mathematical knowledge.