Dyspnea is the most common symptom experienced by patients with chronic obstructive pulmonary disease (COPD) and is linked to decreases in patient activity levels and quality of life. Use of standardized tools to measure dyspnea has a long history in respiratory care. However, in many cases it is not clear what is being quantified when using a particular tool. This review will consider the definition of and mechanisms believed to contribute to dyspnea, attempting to clarify what is being quantified in common tools currently available for dyspnea measurement.