The Next Generation Science Standards (NGSS) is the most recent reform in science education across the United States. The NGSS demands a shift in both teaching and learning. Yet there is no direction on how teachers are to implement this shift in their classrooms. This mixed-methods study examined 12 middle school teachers' perceptions and the instructional practices within the NGSS Science and Engineering Practices (SEPs) by using interviews and classroom observations. Findings suggest that there was a shift in instructional practices and a varying degree of implementation of the eight SEPs. The data analysis identified ongoing needs related to specific professional development. The researcher concluded that district leaders and school principals need to provide tangible supports to teachers in order to successfully meet the demands of this new vision of science education. [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.]