A growing number of American school systems are experimenting with innovative and varied methods of tying educators' salaries more closely to their work through differential pay. Differential pay means paying teachers differently based on their performance, their responsibilities, and/or their teaching assignments. For instance, it can mean increasing pay for teachers who are highly effective, take on the role of master or mentor teachers, or teach in a high-needs school. In some systems teachers' unions are working collaboratively with school districts to develop differential pay programs. For this paper, the author examined four school districts that have successfully partnered with their local teachers' unions to create differential pay programs, in an effort to better understand how these collaborations have evolved and what they are accomplishing. The common elements of these collaborations provide helpful guidance for districts and unions embarking on new reforms of teacher-related policies and programs. (Contains 10 endnotes.)