This dissertation is based on the exploration of the professional development practices of a school and of the role of the principal in ensuring the effective professional development of teachers. The study was carried in a higher secondary school in Karachi, Pakistan. The school might be considered as effective in terms of students' achievements at the Board Examinations. However, it would seem simplistic to attribute the school's success solely to the professional development of teachers. There is a combination of other factors, which were identified as contributing to school effectiveness. The study showed that there are many and varied professional development activities carried out in the school. But professional development is not only carried our through professional development courses or workshops. There are other areas, which can contribute to professional development such as appraisal (identification of teachers' professional development needs), reflective practice and Follow-up. The findings suggest that appraisal is used for two purposes in the school--for identification of teachers' professional development needs, and for evaluation of the teachers' performance. Reflective practice and Follow-up have not received essential attention in the school, and they are practiced on the irregular basis. The principal does a great deal to attempt to ensure effective professional development for his teachers. He encourages and supports professional development of teachers in a variety ways, but the absence of a planned and structured approach to professional development means that, professional development is not so effective as it might be. The study offers some recommendations to school and its principal, which may be useful for ensuring effective professional development. Among these are suggestions to prepare a structured plan of professional development of teachers in the school and to separate two kinds of appraisal. This report also includes the lessons I learned from the study. The findings of the study will be useful for me as a future teacher educator in my native context of Kyrgyzstan, as I found many similarities in organization of the professional development in both systems. I also learned some skills necessary for conducting qualitative research.