The purpose of this study was to examine the effectiveness of an innovative approach to teacher development, the conceptual change approach, that attempts to change teachers' perspectives on assessment. Participants were twelve in-service teachers involved in a staff development program for teachers at the University of Bahrain. The evaluation investigated the program at three levels: The impact of the program on the participants' conceptions of assessment, the resultant impact on their assessment practices, and the consequential effects on their students' gains in achievement. The mixed methods design was used in theory-driven evaluations under the conceptual framework of Program Theory)Ho, 2000(. Results showed that the program brought about detectable conceptual change in four teachers whose students received higher grades in the following year, while none of the students of those teachers who did not change their conceptions about assessment showed similar gains in achievement. Implications for program evaluation theory and assessment practice in schools and higher education institutions are discussed.