In critically ill children, gut dysfunction is frequent, especially gut dysmotility. This is the consequence of various reasons related to critical illness: sedative drugs, mechanical ventilation, immobility, fasting, gut hormone secretion shift, gut circulatory failure and the systemic inflammatory response syndrome. As a result of gut dysmotility, gastrointestinal (GI) symptoms such as delayed gastric emptying or constipation are common in critically ill children. These symptoms should be prevented, diagnosed early and treated proactively. Early enteral nutrition is one of the recommended strategies. Enteral nutrition intolerance may occur, but clinicians must distinguish GI feed related symptoms and non-feed related symptoms. Like any other organ, the gut may be damaged in case of circulatory or hypoxia, but severe complications remain rare in critically ill children. The gut microbiota may also be altered during critical illness. The use of probiotics seems promising to reduce infection and inflammation but large trials are still needed. The aim of this chapter is mainly to address gut dysfunction issues in critically ill children including gut dysmotility, the main gastrointestinal symptoms, and to review the evidence on gut microbiota including the role of probiotics.