This article begins to unravel the question, "What curricular materials work best under what kinds of conditions?" The authors address this question from the point of view of teachers and their ability to implement mathematics curricula that place varying demands and provide varying levels of support for their learning. Specifically, the authors focus on how teacher capacity (their level of education, experience, and knowledge) and their use of curriculum influence instruction. The study sample is 48 teachers implementing two standards-based mathematics curricula--"Everyday Mathematics" and "Investigations"--in two school districts. The data include interviews and surveys with teachers, as well as observations of instruction, over a 2-year period. Findings indicate that teachers' implementation of "Investigations" was considerably better than teachers' implementation of "Everyday Mathematics" in terms of maintaining high levels of cognitive demand, attention to student thinking, and mathematical reasoning. These implementation measures were not correlated to measures of teacher capacity across school districts. However, implementation measures were significantly correlated with teachers' lesson preparation that took into account the big mathematical ideas within curriculum. Further qualitative analysis indicated that the "Investigations" curriculum provided more support to teachers for locating and understanding the big mathematical ideas within lessons compared to "Everyday Mathematics". (Contains 6 tables, 1 figure and 11 notes.)