Research shows that some elementary math curricula are more effective than others at increasing student achievement. Most studies in this research base typically included teachers and classrooms that differ, so the results represent "average" curriculum effects, which raises an important question: Are curricula that are effective on average also effective among different types of teachers and classrooms? We examined whether curriculum effects are moderated by three characteristics that influence curriculum implementation and therefore may influence effects: (a) teachers' knowledge, (b) teachers' attitude toward math instruction, and (c) the extent to which teachers need to differentiate instruction in their classrooms. We examined these moderators for four elementary math curricula that use different pedagogical approaches and specifically for first- and second-grade achievement. We found that the two curricula that were more effective on average also were either as effective or more effective than the other two across all the contexts examined.