The purpose of this study was to explain middle school teachers' decisions for planning a science unit of instruction (reflection on action) and their decisions when enacting this planned unit (reflection in action) using the new teacher professional knowledge and skill (TPK&S) model as a guiding framework. In addition, this study specifically focused on the distinctions between both personal PCK and PCK&S, but also included the entire TPK&S model to help elaborate on and support the peculiarities between personal PCK and PCK&S and what potential factors of the model affect this comparison. A case study research design (Yin, 2013) was employed with three middle school science (8th grade) teachers to examine (1) how these teachers' personal PCK or their planning of a science unit aligns with their personal PCK and skill or their enactment of the planned science unit and (2) what contributes to this alignment between their planning and their practice. Several qualitative based data sources like semi-structured interviews, classroom observations, and lesson plans were collected, analyzed, and triangulated in accordance to this study's guiding framework which primarily consisted of the newly developed teacher professional knowledge and skill model (TPK&S). Results of this study indicate that there was direct alignment between middle school teachers' personal PCK (their planning) and their personal PCK and skill (practice). Three key contributing factors were identified as lending to this direct alignment: their orientations towards teaching science, their prior knowledge, and the context in which they were teaching. Furthermore, the curriculum in which they were using also dominated their planning and practice in such a way that it was the only source of knowledge implemented thus masking both the meaning and importance of the other components of the TPK&S model. This study illustrates the need to understand the factors that play a role in both planning and practice to develop more effective teachers of science. Implications for teacher educators, school administrators, and researchers of teacher knowledge regarding how to address components that affect both planning and practice with both preservice and inservice teachers are 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.]