In this chapter we used developmental dyslexia to exemplify the value of employing longitudinal designs in research on developmental disorders. We show that a full understanding of dyslexia, its characteristics, its causes, and its treatment, depends on studying it longitudinally, and failing to do so would lead to an impoverished account of the disorder. We also make reference to other disabilities as a way of testing the generality and value of our conclusions from dyslexia.