This 15-year program of research explored the extent to which prosocial attitudes and behavior of high school students were increased by focused lesson plans administered in dedicated high school classrooms over 1 or 2 academic semesters. The narrative describes the evolution of the Character Development and Leadership Program from a pilot study in 1 public high school to a curriculum employed by 2,000 high schools nationwide in traditional classrooms and online. First, a Delphi study provided empirically determined consensus about which character traits were most relevant to the needs of educators and students in the high school setting. This was followed by the development and evolution of a focused, highly structured classroom program to inculcate and strengthen these character traits for diverse students in socioculturally diverse high schools. Concurrent efficacy studies suggested that participating students consistently demonstrated a significant diminution of negative behavior outcomes and an increase in positive ones. These were differing kinds of studies in an exponentially growing number of real-life settings. Therefore not all data could be as complete as desired, comparison groups were not always available, and program fidelity was not always constant. Nevertheless, the evolving program and outcomes data make a composite case for the efficacy of the Character Development and Leadership Program and provide a practical illustration for other developers of empirically driven programs in character and leadership education.