Nutritional status is a crucial determinant of clinical outcomes in critically ill patients, influencing complication rates, wound healing, mechanical ventilation duration, ICU length of stay, and mortality. Both obesity and cachexia are associated with adverse prognoses, yet nutritional therapy is often overlooked in routine care. Malnutrition screening remains infrequent in many hospitals, despite strong evidence of its predictive significance. Critically ill patients, especially those admitted as emergencies, are at high risk of undiagnosed nutritional deficiencies. This chapter discusses specific nutritional considerations for obese and cachectic ICU patients, including definitions, prevalence, and tailored nutritional strategies. Obesity-related insulin resistance complicates metabolic management, while cachexia requires a phase-adapted approach to avoid both under- and overfeeding. Nutritional therapy must be carefully monitored to prevent complications such as sarcopenic obesity and refeeding syndrome. Evidence-based guidelines advocate early enteral nutrition, controlled caloric intake, and individualized protein supplementation. Understanding these complexities is essential for optimizing the care and prognosis of critically ill patients with obesity or cachexia.