Parents of children with special educational needs (SEN) typically have concerns both for the present and the future. Practitioners are well used to being asked to predict how the child will develop later during their school careers and also as an adult. In the past, there was a higher level of certainty expressed by professionals, particularly when the child had severe SEN. For example, general cognitive ability as measured by IQ was considered to be stable and hence a good predictor. Over recent decades, this view has altered as evidence for variation in assessed intelligence and developmental changes has been gathered (Hindley and Owen, 1978). It is now clear that children with apparently similar types of difficulties may differ in their developmental trajectories (Leonard, 1998). To provide parents, young people and practitioners with the ability to consider learning needs and developmental trajectories, it is important to gather information from a range of sources. Longitudinal data provide information to address these issues.

The prognosis for children with developmental language difficulties has previously been shown to be a cause for serious concern. A longitudinal study in England of a group of children with receptive language difficulties followed up from age 7–8 years through to their mid-20s and their mid-30s suggested a poor prognosis (Mawhood et al., 2000; Clegg et al., 2005; see Rutter, Chapter 7 this volume). Longitudinal studies by Beitchman and