["Autism Research, EarlyView. ", "\nABSTRACT\nDevelopmental regression during childhood is inconsistently defined and measured, resulting in delayed diagnostic discovery and intervention. This study aimed to examine published definitions and measures for developmental regression. A comprehensive search strategy was applied to Medline, Embase, Cochrane, and PsycINFO databases. Reviewers independently screened relevant studies according to the Preferred Reporting Items for Systematic reviews. The number and percentages of definition use, their primary features, and measures were reported. Of 19,099 potential publications, 157 were included. Thematic analysis identified four condition groups: Neurodevelopmental disorders (n = 118); Progressive neurodegenerative conditions (n = 17); Developmental epileptic encephalopathies (n = 8); and Genetic conditions (n = 14). Most studies (n = 124, 79.0%) used an operational definition, and most specified types of skills lost (n = 147, 93.6%). Age of onset was specified in 67.5% (n = 106), and duration was specified in 34.4% (n = 54). Measures designed to specifically assess developmental regression were infrequently used (n = 5, 3.2%). We concluded that developmental regression is inconsistently defined and measured. A consistent approach is vital to ensure that all children are identified as early as possible. This will allow timely diagnostic discovery, enhance research rigor, and promote the collaboration that is needed to advance our understanding of causal mechanisms and effective interventions for affected children and their families.\n"]