This qualitative Interpretative Phenomenological Analysis (IPA) explored the experiences of teachers who a) participated in a multiyear, multisite professional development (PD) program on formative assessment practices and who b) are identified by their supervising administrators as positive-outliers by virtue of their extraordinary improvement and performance with regards to their formative assessment practices. Black and Wiliam's (2009) theory of formative assessment was used as a lens through which to examine the experiences of these teachers in the PD program. Their experiences were analyzed in terms of the five integrated strategies in Black and Wiliam's theory of formative assessment. A literature review on formative assessment and assessment leadership also informed this study. The data sources were semi-structured interviews with eight teachers. Data was coded using thematic coding and 28 initial codes were distilled into five major themes corresponding to the five integrated strategies in the theoretical framework. The findings of this study have the potential to inform the development of PD on formative assessment for K-12 teachers. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]