During ALS, malnutrition may occur. The causes are multiple. A reduction in caloric and protein intakes, due to swallowing disorders, play a major role. Moreover, (2/3) of patients develop an hypermetabolism of unknown cause, leading to an increase in resting energy expenditure. Malnutrition, whose prevalence is 10-55 percent, is an independant significant prognostic factor for survival. Nutritional assessment is recommanded every 3-6 months at least, and more often if needed. After a simple clinical examination, nutritional assessment includes the measurement of weight (W) and height (H), in order to calculate the Body Mass Index (BMI=W/H(2)). BMI<18.5 (age: 18-65ys) or<20kg/m2 (age>65ys) indicates malnutrition. A weight loss more than 5 to 10 percent of usual weight in the last six months is another malnutrition criterion. The measurement of skinfolds and the calculation of mid arm muscle circumference give informations on respectively fat mass and fat-free mass, but their interest is mainly during the follow-up, or for triceps skinfold when included in a validated bioimpedance formula. Bioimpedance analysis (BIA) is a simple bedside technique, recently validated for measurement of fat-free mass in ALS patients. The BIA phase angle could be an easy obtained parameter of severity. Dietary interview is important when patients still use oral route, but is difficult if they have elocution or writing problems. Indirect calorimetry and dual X-ray absorptiometry give reliable informations on respectively energy expenditure and body compartments, but remain scarcely used. Additional methods, like swallowing tests, psychological, digestive or respiratory assessments are often useful for optimal nutritional prescriptions. We recommand as minimal nutritional assessment a systematic clinical examination, anthropometric measurements and bioimpedance evaluation.