Nowadays, there are two definitions about emaciation at home and abroad. If body weight is lower than 10% standard weight of normal population, it is called emaciation. Generally, many low-weight people don’t have any disease. So a new definition is made: below 10% standard weight of normal population is called emaciation, lower than 20% standard weight can be called as significant weight loss. The other definition is comparing with previous weight. In physiological or pathological condition, weight loss exceeds 5% of previous weight in 6–12 months is called involuntary body weight loss or unintentional body weight loss. Weight is affected by genetic, nutrition, digestion-absorption, consumption and so on. There are low weight children, which is not definitely a pathological state. In clinical practice, most time we should compare with patients’ previous weight.