The origin and developmental course of stereotypic and self-injurious behaviour among individuals with developmental disabilities such as intellectual disability (ID) or pervasive development disorders such as autism is not well understood. Twelve studies designed to document the prevalence, nature, or development of stereotypic and/or self-injurious behaviour in children under 5 years of age and identified as at risk for developmental delay or disability were reviewed. Comparisons were made with similar studies with typically developing children. It appears that the onset of naturally occurring rhythmic motor stereotypies is delayed in young at-risk children, but that the sequencing may be similar. A very small database, differences in samples, measures, and designs limited the degree to which comparisons could be made across studies. Future work is needed based on appropriately designed prospective comparison studies and uniform quantitative measures to provide an empirical basis for new knowledge about the early development of one of the most serious behaviour disorders afflicting children with ID and related problems of development.