This dissertation reports the results of an exploratory case study utilizing quantitative and qualitative methodologies intended to ascertain the extent and differences of implementation of research-based instructional practices, learned in an intensive 26-month professional development, in their urban classrooms. Both the extent and differences in the implementation of practices were investigated in relation to the lesson design and implementation, content, and classroom culture aspects of research-based practices. Additionally, this research includes the concerns of the teachers regarding the factors that helped or hindered the implementation of research-based practices in their classrooms. Six graduates of the Master of Chemistry Education Program who were teaching a chemistry course in a high school in the School District of Philadelphia at the time of the study (2006-8), were the case. The teachers completed a concerns questionnaire with closed and open-ended items, and rated their perceptions of the extent of implementation of the practices in their urban classrooms. Additionally, the teachers were observed and rated by the researcher using a reform-teaching observation protocol and were interviewed individually. Also, the teachers submitted their lesson plans for the days they were observed. Data from these sources were analyzed to arrive at the findings for this study. The research findings suggest that the group of teachers in the study implemented the research-based practices in their classrooms to a low extent when compared to the recommended practices inherent to the MCE Program. The extents of implementation of the practices differed widely among the teachers, from being absent to being implemented at a high level, with inconsistent levels of implementation from various data sources. Further, the teachers expressed the depth of knowledge (gained in the MCE Program), formal laboratory exercises and reports, administrative support, self-motivated students, and group/collaborative work as several factors that enabled or would have enabled the implementation of practices. Among the many factors that hindered the implementation of the practices in their urban classrooms were, the core curriculum and pacing schedule, followed by test preparation, administrative paper-work, large class-size, students not prepared for student-centered work, poor math and reading skills of students, students' lack of motivation, unsupportive department head, unresponsive administration, and lack of resources.