Portfolio assessment of teaching practice was first introduced into the University of Botswana's Faculty of Education in May 2010. This study sought to investigate the impact of this innovative new professional development/assessment system on the professional growth and development of in-service teachers. The findings of the study suggest that although student teachers found portfolio development to be cumbersome and time-consuming, the general consensus was that developing portfolios provided useful and valuable learning experience which enhanced their creativity, reflective practice and continuous professional growth. Overall, students found portfolio development and its assessment and evaluation of a cumulative collection of their creative works to be a better approach for the assessment of teaching than the traditional approach. The study also found out that there was a lack of consistency in the way supervisors assessed students' portfolio and for this reason, it concluded with the recommendation that in order to increase the reliability and credibility of portfolio assessment, there is a need to provide intensive orientation and clear guidelines to students and all classes of supervisors in future in the form of clear expectations for the purpose of selection of artifacts, organization, reflection, assessment and evaluation of portfolios. (Contains 1 table.)