Dyspnea is frequently a multicausal and devastating symptom among advanced cancer patients. It occurs in 21%-78.6% of patients days or weeks before death and is often difficult to control. The genesis and pathophysiology of dyspnea as a symptom still has not been well understood. Dyspnea is frequently associated with abnormalities in the mechanisms that regulate normal breathing; however, the actual expression of dyspnea by a patient results from a complex interaction between the abnormalities in breathing and the perception of those abnormalities in the central nervous system. The production of dyspnea has to be related to the activation of mechanoreceptors both in the respiratory muscles and in the lung, even in the absence of increased muscle respiratory activity. Respiratory muscle weakness appears to be an important cause of dyspnea in malnourished, asthenic, and cachectic cancer patients. This might also explain why about 24% of dyspneic cancer patients do not present cardiac/pulmonary disease. In addition, two other possible mechanisms of dyspnea have been proposed: chemoreceptor stimulation and efferent activity from the respiratory center by direct ascending stimulation. These factors and the assessment tools used in patients with chronic dyspnea are summarized in this review.