The research objectives addressed in this dissertation include: the measurement of the effects of an inquiry-based middle school science professional development program on teachers, the development and utilization of a student-level scientific reasoning ability test, and an examination of the connection between the scientific reasoning ability of teachers and that of their students. The effect of the inquiry-based science professional development on teachers was determined through pre- and post-test comparisons of scientific content tests, a scientific reasoning ability measure, and a self-reported survey of teacher classroom practices. Analyses showed significant increases on the level of scientific content knowledge as well as significant improvement in scientific reasoning ability. Pre- and post-test results of the classroom practices survey revealed a move from teacher-centered to student-centered teaching methods. A measure of scientific reasoning ability was developed and administered to middle-school students, and the results were analyzed using Rasch modeling. Analysis revealed a good fit to the Rasch model as well as to levels of cognitive demand within areas of scientific reasoning ability. Student abilities as estimated by the Rasch analysis were significantly higher from the time of the pre-test to the time of the post-test. Through the use of hierarchical linear modeling, a relationship between a teacher's ability to reason scientifically and the end-of-year reasoning ability scores of his or her students was established. Results indicated a significant, positive relationship between a teacher's scientific reasoning ability and that of his or her students. [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.]