First described over 100 years ago, developmental dyslexia (reading disability) has been a controversial subject. There has been much confusion regarding the definition, the psychological basis, the conceptualisation and even the name. Recent research has helped us to understand the main underlying cognitive deficits that lead to poor reading. Brain imaging studies conducted during the performance of reading tasks have shed light on its biological basis and genetic studies have identified a number of genes that are associated with reading difficulties. Together, these studies have led to a redefinition and new understanding of developmental dyslexia. This article examines these recent findings. A second article to appear in Advances will focus on clinical aspects of the disorder.