The term asthenia comes from the Greek (centsqsneia, a: privation, without; esthénos: vigor, force), it means absence of strength, vigor or force. It is a symptom, difficult to define, with a set of vague sensations, different for each patient. It is a frequent cause of consult, almost 30% in ambulatory settings. The chronic fatigue represents up to 10% of these cases, and the 0.2-0.7% belongs to the chronic fatigue syndrome. It is very important to differentiate asthenia from weakness, dizziness or dyspnoea, since patients may confuse them. The factor time in asthenia is very useful for its characterization, it was defined to the prolonged fatigue when it lasts for more than a month and chronic when the duration is greater than 6 months. The depression is the commonest fatigue cause, representing approximately half of the cases. The most effective treatment of the asthenia is to solve the underlying cause, although up to 20% of the patients remain without diagnosis. The diagnosis of the chronic fatigue syndrome is of exclusion and the criteria of the international consensus of year 1994 are due to use. The high frequency of the symptom entails an enormous social and economic cost and it is for that reason so important for physicians to have a correct manage of this symptom.