The objective of this work was to develop guidelines for nutritional support in critically ill adults and children (excluding neonates and burn patients) unable to maintain an adequate oral intake. We aimed to provide up-to-date recommendations based on high-level evidence including the results of recent landmark randomized controlled trials. Experts from the French Intensive Care Society (SRLF), the French Society of Clinical Nutrition and Metabolism (SFNCM), and the French-Speaking Group of Pediatric Emergency Physicians and Intensivists (GFRUP) used the GRADE methodology to develop the guidelines. Twenty-four Patient Intervention Comparator Outcome (PICO) questions were identified, resulting in 34 adult and 29 pediatric recommendations. Of the 34 recommendations for adults, three were based on high-level evidence, 12 on moderate-level evidence, and 19 on expert opinion. The corresponding numbers for the 29 pediatric recommendations were one, five, and 23. All recommendations achieved strong agreement among the experts. These guidelines emphasize the importance of individualized nutritional support strategies that incorporate recent high-quality evidence to optimize the outcomes of critically ill patients.