The studies reported in Chapter 4 clarify the nature of the short-term memory deficits associated with severe learning difficulties. Descriptively, we can say that in severe learning difficulties, whether attributed to Down’s syndrome or a variety of other pathologies, verbal short-term memory span shows a marked failure to develop. This failure of development leads to an increasing lag between mental age and short-term memory skill as these children get older. This pattern is quite striking when it is remembered that mental age is, itself, developing at a slow rate in these subjects. It remains to try to specify, in terms of models of memory processes and their development, the origins of this failure of short-term memory development in severe learning difficulties.