BACKGROUND & AIMS:Clinicians and researchers often use feeding intolerance (FI) as main cause for insufficient enteral nutrition (EN). However, there is no uniform definition for FI. A uniform definition is essential for future studies focusing on predictors and outcomes of FI and enteral nutrition. A systematic review was performed to investigate the definitions, prevalence, predictors and outcomes of FI in critically ill children. METHODS:The databases Medline, Embase, Cochrane CENTRAL, Web of Science were searched. Inclusion criteria were interventional, observational or case-control studies (>10 patients) in which a definition of FI was reported in critically ill children (0-21 years). RESULTS:FI was defined in 31 unique studies performed in 2973 critically ill children. FI was most commonly defined as presence of gastrointestinal (GI) symptoms and/or large gastric residual volume (GRV) (n = 21), followed by discontinuation of EN due to GI symptoms (n = 7) and inadequate delivery of EN (n = 3). Median prevalence of FI was 20.0% [IQR 7.4%-33.0%]. Large GRV, abdominal distention, diarrhoea and vomiting/emesis, were the predominantly reported GI symptoms to define FI. FI was associated with severity of illness, mortality and nosocomial infections. CONCLUSIONS:Feeding intolerance is inconsistently defined in the current literature, but appears to be a prevalent concern in critically ill children. FI is most frequently defined by the presence of GI symptoms. A standardized definition is needed for both clinical and research purpose to determine the consequences of FI in relation to short-term and long-term outcomes. The new proposed definition for FI entails the inability to achieve enteral nutrition target intakes in combination with the presence of GI symptoms indicating GI dysfunction. PROTOCOL REGISTRATION:PROSPERO registration number: CRD42018092967. Registered on 07 June 2018.