Although interim assessments are currently promoted as a mechanism for improving teaching and student learning, we know little about how teachers use this data to modify instruction. This article presents findings from a larger study on teachers' use of interim assessment information in elementary mathematics. We address the following questions: (a) How do the Philadelphia teachers in our sample analyze benchmark assessment results, (b) how do they plan instruction based on these results, and (c) what are their reported instructional responses to such results? To answer these questions, we interviewed all 3rd- and 5th-grade teachers in five average- and above-average-performing elementary schools three times during the 2006-07 school year. We found that although the teachers in our study used interim assessment results to gain information about students' learning in mathematics, teachers did not use interim assessments to make sense of students' conceptual understanding. Furthermore, teachers' tendency to interpret student errors as procedural missteps was paralleled by a trend toward procedural instructional responses. (Contains 1 figure and 2 footnotes.)