Nutrition is a crucial component of critically ill patients' treatment. The key questions to address are when to initiate nutrition therapy, how to determine the optimal route, the appropriate amounts of macro and micronutrients, and the precise energy requirements for each patient. Critical illness has three phases: early period (previously "ebb phase") lasting 1-2 days; late period (previously "flow" phase) lasting for 3-7 days; late phase (Phase Rehabilitation or Chronic Phase). Each of the above-mentioned phases has its characteristics. During the first phase, severe catabolism is increased, and it gradually proceeds to anabolism during the following 3 to 4 days. The recommendations for critically ill patients' nutrition have been formed based on these phases. Early nutrition therapy, especially early parenteral nutrition with high energy and protein intake, should be avoided in the first three days of critical illness. Reaching the nutritional goal should be initiated only 3 to 4 days upon the onset of critical illness. According to ESPEN recommendations, daily calorie intake should be initiated at 20-25 kcal/ kg/day, while daily protein intake should be initiated at 0.8g/kg/day with a gradual increase to 1.3 g/kg/day. On the other hand, ASPEN recommends 12-25kcal/kg/day of daily calorie intake with daily protein intake at 1.2-2 g/kg/day. The optimal route of feeding is enteral whenever possible. Alternatively, parenteral route should be used. Indirect calorimetry serves as a basis for determining nutritional needs in critically ill patients. Conclusion: Nutritional therapy is essential for critically ill patients. Nutrition should be obtained through enteral route whenever possible. Energy and protein intake should be gradually introduced in critically ill patients' treatment. Nutritional therapy prescription should be adapted to the patients' needs.