The purpose of this mixed-methods study was to gain insights and understandings of high school teachers' perceptions and use of formative assessment to enhance their planning, individualization of instruction, and adjustment of course content to improve student learning. The study was conducted over two years in a midwestern high school of approximately 1,000 students. Crucial to the three project teachers' understanding of formative assessment was developing and using preset curriculum road maps that tightly aligned course goals, learning objectives, activities, instructional methods, and assessment. The in-depth case studies of the sample's three teachers revealed that, when provided with specific information about formative assessment through staff development, they became more positive toward such assessment, and their implementation skills were greatly improved. The staff development had an especially positive impact on the teachers' understanding and skill sets for individualizing instructional practices. The personalization of the staff development proved to be the most beneficial when it tailored the content to the varying levels of initial proficiency of the three sample teachers. Support for formative assessment by the administrative team members was essential to creating a cultural shift from summative to formative assessment.