This mixed methods study explored how elementary teachers reported using a commercially published instructional unit to plan and deliver science instruction in their elementary classrooms. Of particular focus is what elements of the program teachers eliminated during planning and instruction, what outside materials teachers added to instruction, what modifications they made to the materials, and their rationales for these changes to the prescribed program. This study consisted of two phases. The first was a survey of elementary science teachers who taught the "Smithsonian Science for the Classroom" units. Data collected from this phase informed the subsequent interview phase. I used deductive coding to analyze the data and a descriptive narrative format to report the findings and implications of this study to answer the following questions: How do teachers report using instructional materials in elementary science during the planning and delivery of instruction? What modifications, if any, do teachers report making to instructional materials? What rationales do teachers report for their modifications? Two clear themes emerged from the study. First, teachers eliminated lessons and shortened tasks to fit instruction into the minutes designated for teaching science. Time constraints or the perception by teachers that the tasks would be too difficult for their students were typical rationales given for making any changes to the materials. Implications for teachers, administrators, professional development providers, and policymakers were discussed. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]