Tomlinson (2000) outlined four general classroom elements that can be differentiated (p.1):
(1) Content— what the student needs to learn or how the student will get access to the information;
(2) Process—activities in which the student engages in order to make sense of or master the content;
(3) Products—culminating projects that ask the student to rehearse, apply, and extend what he or she has learned in a unit; and
(4) Learning environment—the way the classroom works and feels.
Of these four categories, teachers generally recognize that the majority of their professional teaching preparation programs have focused on one area: content. When hired and assigned to teach respective areas of content, most teachers encounter a wide range of student responses to the instruction being provided. These challenges are now well-documented, and teachers report not feeling adequately trained to fully anticipate and address the learner variance they encounter. This data serves to highlight that teacher preparation programs do not always provide sufficient opportunity for teacher candidates to reflect on principles and practices that would foster development in the areas of process, products, and learning environments.