This thesis explores by means of a small intervention study based on ordinary clinical practice, the effectiveness and acceptability to carers of an intensive behavioural modification approach to the chronic and severe night-settling and night-waking (NS/NW) sleep problems commonly encountered with severely learning disabled (SLD) children. The principal hypotheses tested were: (i) rapid settling and extinction techniques represent an effective means of treating severe and chronic NS/NW sleep problems presenting in severely learning disabled children; (ii) such an approach will also provide useful generalization effects; (iii) such an approach will prove acceptable to the parents of affected children; and (iv) electronic ambulatory movement monitoring (AM) will yield useful data in this context. Fifteen sleep disordered SLD children were recruited into the trial and there were no drop-outs over its course. The results of the study reflect that the intervention was highly successful and that improvements in the children's sleep problems were evident from subjective and objective assessments within a few days of the commencement of treatment. At long-term follow-up, these improvements were sustained in all but three of the families. Generalization effects were seen in that the children's daytime behaviour improved as did the indices of their mothers' subjective wellbeing. The parents were highly satisfied by both the results and the style of the interventional approach. No adverse effects were reported. The AM data confirmed the trends of improvement detected in the parent and investigator assessments (night-waking patterns; settling times; etc.) and also provided novel information about the duration, movement content and temporal distribution of night-waking episodes over the course of the study for five of the children. These findings are discussed in terms of their implications for clinical practice and future research.