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32 Chapter 1 ABSTRACT Objective: To systematically review the literature regarding the epidemiology of functional constipation (FC) and functional nonretentive fecal incontinence (FNRFI) in children. Secondary objectives were to assess the geographical, age and sex distribution of FC and FNRFI and to evaluate associated factors. Study design: The Cochrane Library, PubMed and EMBASE databases were searched from 2006 until September 2017. The following inclusion criteria were applied: (1) prospective studies of population-based samples; (2) reporting on the prevalence of FC or FNRFI according to the Rome III/IV criteria; (3) in children 0-18 years of age; (4) published in full manuscript form. A quality assessment of included studies was conducted. Random effect meta-analyses with meta-regression analyses of study characteristics were performed. Results: Thirty-seven studies were included of which 35 reported on the prevalence of FC and 15 on the prevalence of FNRFI. The reported prevalence of FC ranged from 0.5% - 32.2%, with a pooled prevalence of 9.5% (95% CI 7.5-12.1). The prevalence of FRNFI ranged from 0.0%-1.8%, with a pooled prevalence of 0.4% (95% CI 0.2-0.7). In studies describing sex-distribution, the prevalence of FC was 8.6% in boys compared to 8.9% in girls (OR 0.99, 95% CI 0.9-1.4). Geographical location, dietary habits and exposure to stressful life events were reported to be associated with the prevalence of FC. Data on FNRFI were scarce and no associated factors were identified. Conclusion : FC is common in childhood and the prevalence of FC is associated with geographical location, lifestyle factors and stressful life events. FNRFI is rare and no associated factors were identified.