The purpose of this article is to address a major gap in the instructional sensitivity literature on how to develop instructionally sensitive assessments. We propose an approach to developing and evaluating instructionally sensitive assessments in science and test this approach with one elementary life-science module. The assessment we developed was administered to 125 students in seven classrooms. The development approach considered three dimensions of instructional sensitivity; that is, assessment items should: represent the curriculum content, reflect the quality of instruction, and have formative value for teaching. Focusing solely on the first dimension, representation of the curriculum content, this study was guided by the following research questions: (1) "What science module characteristics can be systematically manipulated to develop items that prove to be instructionally sensitive?" and (2) "Are the instructionally sensitive assessments developed sufficiently valid to make inferences about the impact of instruction on students' performance?" In this article, we describe our item development approach and provide empirical evidence to support validity arguments about the developed instructionally sensitive items. Results indicated that: (1) manipulations of the items at different proximities to vary their sensitivity were aligned with the rules for item development and also corresponded with pre-to-post gains; and (2) the items developed at different distances from the science module showed a pattern of pre-to-post gain consistent with their instructional sensitivity, that is, the closer the items were to the science module, the larger the observed gains and effect sizes. (Contains 3 figures, 6 tables, and 3 notes.)