While nutrition therapy in critically ill patients has been improved in decades, continuous efforts to reach optimal nutrition therapy for each individual patient are still being made. Nutritional monitoring and assessment are important to analyze and understand the challenges of heterogeneity in critical ill patients. Measurable indicators of biological processes, pathogenic states, or metabolic responding to nutritional interventions can guarantee the achievement of optimal nutrition goal. More effective guidance to individualized nutrition therapy is to understand the interplay of pathophysiologic changes between nutrition, gastrointestinal tract, and critical illness. This article focuses on nutrition monitoring in critical ill patients during nutrition therapy and some nutritional biomarkers, and also discusses the feasibility and availability in the critical care setting.