This report provides an overview of the development, implementation, and psychometric properties of a student mathematics interview designed to assess first- and second-grade student achievement and thinking processes. The student interview was conducted with 622 first- or second-grade students in 22 schools located in two public school districts in a single state in the southeastern U.S. during spring 2014. Focused on the domain of number, operations, and equality, the student interview was designed (a) to measure student achievement in mathematics and (b) to gather information about the strategies students use to solve the mathematics problems. Because the interview was designed for both of these purposes, we call it the Mathematics Performance and Achievement (MPAC) interview.