To some teachers, the term "differentiation" means to do different things for the students who need remediation or who have disabilities. To other teachers, differentiation refers to any provision for student differences, including for those students who are more capable than average. There are also many teachers who do not use differentiation at all. The reasons for not using it are familiar to everyone: too many students, not enough time, and lack of know-how. The author decided to investigate, through informal means, factors that encourage and enable those teachers who do differentiate. In seeking to determine what teachers know about differentiation, he interviewed and observed teachers and graduate students. Interviews and questionnaires reveal why some teachers differentiate and why some do not.