Dyspnea, an unpleasant sensation of difficulty in breathing, is a common accompaniment of cardiopulmonary disease. The underlying mechanisms generating this sensation are not clearly understood. There does not appear to be any one specific site or specific receptor(s) involved in this sensation; however, reflex increase in central respiratory motor "command," as well as activity of the respiratory muscles, appear to be necessary for the genesis of the sensation. Whether there is a direct dyspnogenic effect of changes in chemical drive (increased arterial PCO2 or decreased arterial PO2) is unclear. Several methods to quantify dyspnea for clinical purposes have been described; techniques using exercise as the stimulus and expressing the response on a visual analogue or Borg category scale appear to be clinically applicable. The specific treatment of dyspnea remains in the experimental stage. The direct effects of exercise conditioning are unclear. A number of drugs (mainly central nervous system depressants) have been examined; preliminary work holds promise, but no particular drug can as yet be recommended for routine clinical use.