Using data collected in 125 seventh-grade and 56 eighth-grade Texas classrooms in the context of the "Scaling Up SimCalc" research project in 2005-07, we examined relationships between teachers' mathematics knowledge, teachers' classroom decision making, and student achievement outcomes on topics of rate, proportionality, and linear function--three important and cognitively demanding prealgebra topics. We found that teachers' mathematical knowledge was correlated with student achievement in only one study out of three. We also found a lack of correlations between teachers' mathematical knowledge and critical aspects of instructional decision making. Curriculum and other learning resources (e.g., technology, student-student interactions) are clearly important factors for student learning in addition to, and in interaction with, teachers' mathematical knowledge. Our results suggest that mathematics knowledge for teaching may have a nonlinear relationship with student learning, that those effects may be heavily mediated by other instructional factors, and that short-term content knowledge gains in teacher workshops may not persist in classroom instruction. We discuss a need in the field for richer models of how "mathematical knowledge for teaching" works in the context of complete instructional systems. (Contains 7 tables and 6 figures.)