We review the evolution of the conceptualisation of dyslexia and along with it the current, 21st century definition of dyslexia. Our starting point is the seminal report by Pringle Morgan in 1896, followed by early 20th century reports by Hinshelwood, and continuing with concepts of brain injury and minimal brain dysfunctions then to the emergence of 'learning disability' in the mid-20th century. Within this historical framework we note the emergence and scientific validation of an 'unexpected' model culminating in the first definition of dyslexia in U.S. federal law in late 2018 and in recent U.S. federal civil rights law. We next address the issue of whether dyslexia is 'real' by showing how advances in non-invasive functional brain imaging have led to a replicated, robust neural signature for dyslexia. We close by illustrating how science has informed policy, emphasising the focus on evidence-based practices beginning with the report of the National Reading Panel and including the creation of the What Works Clearinghouse. Early screening for dyslexia represents another important policy accomplishment with dyslexia laws now passed in over 30 states. Evidence-based effective interventions for dyslexia, including free, public charter schools specialised for dyslexic children exemplify the most recent policy initiative.