Breathlessness or dyspnea-the subjective experience of breathing discomfort-is a symptom in many pulmonary, cardiovascular, and neuromuscular diseases. It occurs in normals as well during intense emotional states and heavy labor or exercise. In clinical cases, it generally causes severe suffering. Dyspnea has multifactorial causes and the explanation for the symptom may differ largely among patients. Explanatory models imply the involvement of mechanisms at several levels of functioning, such as afferent signals from the respiratory muscles or blood gas levels related to hypercapnia and hypoxia. Depending on the relative involvement of specific mechanisms and their interactions, dyspnea may be experienced differently and subtypes can be distinguished. More recently, perceptual-cognitive and emotional processes related to symptom perception and interpretation have been investigated in the context of dyspnea.