In the last decade the term ‘comorbidity’ has gained popularity in the field of paediatric neurodisability, with the increasing recognition that many conditions are rarely present in isolation. Within this field, the term is often used to refer to the co-occurrence of conditions more frequently than would be expected by chance, which can include instances where one condition causes the other, where they share a common cause (for example, genetic), or where they are in fact manifestations of a single condition. Whether it is valid to use the term ‘comorbidity’ in all these situations, and how precisely it should be used, is something that the contributors to this book grapple with in their own fields of interest. The contributors, all world experts in their fields, also discuss what we can learn from the presence of comorbidities, however defined, about the aetiology and treatment of neurodevelopmental disabilities. In particular, they demonstrate how our increasing understanding of the mechanisms underlying the common association of many ‘comorbidities’ is helping us to understand the natural history of these conditions and improve our management of them.