Why is it important to detect and define learning disability in a child, and how can one do so? The answers to the first question are, largely, not in dispute. Not only does the recognition and definition of learning disability contribute to our understanding of the basis of problems in a child's educational, developmental, and social progress, it also has significant long-term consequences, especially where the disability is more severe. The diagnosis or medical aetiology of the child's learning disability is a significant element of the story and carries major phenotypic implications, in terms of dysmorphology, organogenesis, development, and behaviour. It is one thing to diagnose, say, Down syndrome, and quite another to reach an accurate appraisal of the individual child's intellect, developmental attainments, and capacities. In turn, the definition of learning disability is not always straightforward and is sometimes quite controversial. This is partly because, traditionally, at least three different approaches are used to define learning disability in children, and each has its inherent attractions and pitfalls. These approaches are: definition by IQ alone; definition by IQ combined with social functioning and age at onset; and definition by service contact, the so-called ‘administrative definition’.