Aggressive nutrition therapy is essential to improve nutrition and function in patients with malnutrition and sarcopenia. Malnutrition and sarcopenia negatively affect functional recovery and activities of daily living. Nutrition improvement is associated with better functional recovery. Target energy intake in aggressive nutrition therapy is defined as total energy expenditure (TEE) plus the amount of energy accumulated. The amount of energy accumulation per 1 kg of body weight is generally 7500 kcal. If the goal is to gain 1 kg of weight over 30 d, TEE + 250 kcal is the target daily energy intake. Aggressive nutrition therapy is implemented using a rehabilitation nutrition care process, which consists of five steps: assessment and diagnostic reasoning, diagnosis, goal setting, intervention, and monitoring. Aggressive nutrition therapy sets clear goals using the Specific, Measurable, Achievable, Relevant, and Time-bound principles. The application and effect of aggressive nutrition therapy differs depending on the etiology and condition of malnutrition. Precachexia, short bowel syndrome, and older people with mild to moderate dementia are indications for aggressive nutrition therapy. Nevertheless, aggressive nutrition therapy is usually contraindicated in cases of refractory cachexia, acute disease or injury with severe inflammation, and bedridden patients with severe dementia and reduced activity. Aggressive nutrition therapy should be combined with aggressive exercise and rehabilitation. Enhanced nutritional therapy combined with rehabilitation in patients with cerebrovascular disease, hip fracture, or acute disease is recommended in the 2018 clinical practice guidelines for rehabilitation nutrition. Further evidence for aggressive nutrition therapy is however required.