This is a qualitative, descriptive, and multi-case study, in which the researcher investigated the perceptions of middle school teachers who were implementing three-dimensional learning aligned with the Next Generation Science Standards (NGSS) into their instructional practices. The participants were 10 middle school teachers who worked in three urban middle schools in western Connecticut. Data were gathered using survey questions, classroom observations using the EQUIP rubric version 2.0, and semi-structured interviews. The research questions were: What do middle school teachers believe is important for the teaching of science, when implementing the Next Generation Science Standards? What are the perceptions of middle school teachers regarding the strengths and weaknesses of the science curriculum after the adoption of the Next Generation Science Standards? How should professional development support middle school teachers who are implementing the Next Generation Science Standards? Themes that emerged through analyses of data included barriers and catalysts, development and methods. Results indicated that ongoing and focused professional development provides mastery experiences that lead to higher levels of teacher self-efficacy. Professional development created a clear structure for collaboration and this increased the participants' levels of motivation for implementing three-dimensional learning. Semi-structured interview data indicated that trained teachers teaching colleagues on an ongoing basis in addition to support from school and district administration was a catalyst to implementing three-dimensional learning. Qualitative results identified three sources as impacting self-efficacy: vicarious experiences; coaching and feedback; and performance outcomes or past experiences. Interview data indicated that the collective impact of both administrative support and collegial support not only encouraged teachers to change routine teaching practices and to take risks but it also provided supports to increase teacher self-efficacy and outcome expectancy. [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.]