Approximately two thirds of cancer patients at advanced stages of the disease suffer from anorexia, which leads to significant weight loss and progressive cachexia, an important factor that contributes to death. It has been observed that cancer cachexia differs from simple starvation, although the exact mechanisms associated with cancer cachexia are not well known. Several theories regarding its pathogenesis point to a complex mixture of tumor, host and treatment variables. Unfortunately, the wasting syndrome also constitutes for the patient, a progression of the cancer process, significantly affecting quality of life and social interactions. Treatable causes should be identified and treated. Knowledge of the mechanisms underlying the effects of caquexia on the patient may play a role in identifying treatment measures targetted to muscle wasting and to maintain body strength. In this article we review the main features and mechanisms of the anorexia-cachexia syndrome in patients with cancer.