Teaching science through inquiry-based methodologies has been advocated by several governmental and professional organizations (National Research Council [NRC], 2000; NGSS Lead States, 2013); however, their use is not widespread. Why is this? What science teachers do in their classrooms is shaped by their beliefs, especially their beliefs about the nature of science, about the goals and purposes of school science and the goals of teaching science, and about the teaching and learning science. By critically examining these core conceptions and their influence on teacher lesson planning, this study aims to shed light on the role conceptions play when teachers design their lessons within the context of a specific inquiry-based methodology--the 5-DIE instructional methodology. A total of five middle school science teachers participated in this case study. The findings of this study revealed that teachers' core conceptions--their conceptions about the nature of science, the goals and purposes of middle school science, the goals of teaching science, teaching science, and learning science exhibited coherence among themselves. This study also found that only the teachers who: (1) held the view that subjectivity is involved in the nature of science; (2) held learner-centered goals; and (3) had student-centered conceptions about science teaching and learning were able design a 5-DIE lesson plan that was true to its inquiry nature. Finally, the results of the current study suggest that teachers' conceptions about the nature of science are not immediately available to them and do not directly surface as teachers reflect and elaborate on their 5-DIE lesson plan design decisions. However, the teachers' conceptions about the goals and purposes of middle school science, the goals of teaching science, and science teaching and learning are readily available and clearly shape their design decisions regarding their 5-DIE lesson plans. [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.]