Background & aims: This position paper summarizes theoretical and practical aspects of the monitoring of artificial nutrition and metabolism in critically ill patients, thereby completing ESPEN guidelines on intensive care unit (ICU) nutrition. Methods: Available literature and personal clinical experience on monitoring of nutrition and metabolism was systematically reviewed by the ESPEN group for ICU nutrition guidelines. Results: We did not identify any studies comparing outcomes with monitoring versus not monitoring nutrition therapy. The potential for abnormal values to be associated with harmwas clearly recognized. The necessity to create locally adapted standard operating procedures (SOPs) for follow up of enteral and parenteral nutrition is emphasised. Clinical observations, laboratory parameters (including blood glucose, electrolytes, triglycerides, liver tests), and monitoring of energy expenditure and body composition are addressed, focusing on prevention, and early detection of nutrition-related complications. Conclusion: Understanding and defining risks and developing local SOPs are critical to reduce specific risks.