The purpose of this study was to determine the extent to which sustained teacher professional development in science education affects the classroom instruction of elementary school teachers in third through sixth grade over a 3-year period. The teachers in the study were all elementary endorsed and prepared to be generalists in the content areas. Science reform has led to more content-specific science standards that are difficult for most elementary teachers to address without professional development. Recent studies on improving elementary science instruction suggest the need for professional development to be long term, embedded in teaching practice in the classroom, and rooted in research on how children learn science. The researcher examined changes in classroom instruction over a 3-year period of teachers who participated in a professional development program designed to meet the elementary science education reform based on recommendations from the National Research Council's report, "Taking Science to School: Learning and Teaching Science in Grades K-8." The data that were analyzed to determine the effects of the professional development came from classroom observations of two sets of teachers, one of which was the control set (n = 20). The other was the experimental set (n =22). Classroom observations were administered one time each year over 3 years of treatment to determine whether sustained professional development in science impacted teacher practices in the classroom. This study suggested that classroom science instruction did significantly change through sustained professional development intervention. It also suggested that teaching practices improved in the areas of talk and argument, investigation and inquiry, modeling and representations, alignment with science core concepts, and addressing science misconceptions. Furthermore, findings indicated that teachers who received sustained professional development were more likely to have higher overall effective science instruction scores. [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/en-US/products/dissertations/individuals.shtml.]