UDL is based upon the most widely replicated finding in educational research: learners are highly variable in their response to instruction. In virtually every report of research on instruction or intervention, individual differences are not only evident in the results; they are prominent. However, these individual differences are usually treated as sources of annoying error variance as distractions from the more important “main effects.” UDL, on the other hand, treats these individual differences as an equally important focus of attention. In fact, when viewed through the UDL framework these findings are fundamental to understanding and designing effective instruction. The research that supports UDL falls into four categories: foundational research of UDL, research on the UDL principles, research on promising practices, and research on implementation of UDL.