Elementary school children in the United States are not developing acceptable levels of mathematical proficiency (National Center for Education Statistics, 1999), and a major concern of teacher educators is that teachers lack the depth and flexibility of mathematical understanding and the corresponding beliefs they need to teach for proficiency (National Research Council [NRC], 2001). Although teachers' mathematical content knowledge plays a critical role in their instruction, teachers need more than content knowledge to be effective. Beliefs about mathematics, teaching, and learning affect not only the ways teachers teach mathematics but also the ways prospective teachers learn mathematics. This article is based upon the author's assumption that, for prospective elementary school teachers, separating the learning of mathematics from the consideration of issues of mathematics teaching and learning is counterproductive to their development of mathematical content knowledge and to the development of their beliefs about mathematics teaching and learning. Here, the author presents the theoretical underpinnings and summarizes the data in support of the claim that prospective elementary school teachers (PSTs) benefit by learning about children's mathematical thinking concurrently while learning mathematics. The author describes four principles that serve as the focus of a mathematics laboratory developed and implemented for PSTs at San Diego State University and at local community colleges: (1) The way most students are learning mathematics in the United States is problematic because students learn to manipulate mathematical symbols without developing the underlying conceptual meanings for the symbols; (2) Learning concepts is more powerful and more generative than learning procedures; (3) Students' reasoning is varied and complex, and it is generally different from adults' reasoning; and (4) Elementary mathematics is not elementary. (Contains 8 figures and 3 notes.)