Context: Changes in the prevalence of chronic health conditions in childhood have considerable societal consequences for health care planning and for employment. To obtain valid and reliable estimates of the prevalence of chronic health conditions, a clear definition is needed. Objective: To present an overview of all definitions and operationalizations that have been applied to measure the prevalence of chronic health conditions in childhood. Data Sources: PubMed and the Web of Science were searched for articles published up to December 2006. Also, references were searched by hand for related articles. Study Selection: Non-English- and non-Dutch-language articles were excluded. Of 7252 articles found, 64 articles that stated a conceptual definition and/or operationalization of chronic health conditions in children (aged 0-18 years) were included. Data Extraction: Data on the (1) definition; (2) operationalization in terms of source of information, method of information retrieval, and study population; and (3) resulting prevalence rate were extracted by 2 independent reviewers. Data Synthesis: A large range of definitions were in use, of which 4 were cited by many authors. Various operationalizations of the concepts that were measured were identified. Chronic health conditions in childhood prevalence estimates ranged from 0.22% to 44%, depending on these operationalizations. Conclusions: The wide variability in reported prevalence rates of chronic health conditions in childhood can be explained by considerable diversity in the concepts and operationalizations used. International consensus about the conceptual definition of chronic health conditions in childhood is needed.