Researchers in developmental disorders frequently refer to abilities that are in line with mental age as simply delayed. The qualifier simply might imply an existing theory of developmental delay that is well understood and uninteresting (perhaps because it is an exaggerated form of individual differences, the responsibility of other researchers). In this article, I argue that the notion of delay can be separated into descriptive and explanatory versions. The descriptive version is often used too coarsely to be helpful. Instead, we need an approach based on developmental trajectories to separate types of descriptive delay, which may then have different underlying causes. The explanatory version is poorly articulated in developmental theory. One useful way to deepen our understanding of delay is by building computational models that simulate development in large populations of individuals and explicitly implementing factors that cause variations in development. Finally, I suggest that dividing research among separate investigators of typical development, individual differences, and developmental disorders may be counterproductive if the underlying mechanisms recognize no such distinction.