The cardiopulmonary exercise test (CPET) permits the most accurate and reproducible quantification of cardiopulmonary fitness, a grading of the etiology and severity of cardiopulmonary impairments, and an objective assessment of the response to an intervention. Moreover, over the last three decades, a large volume of research has been directed toward the utility of CPET as a diagnostic and prognostic tool; these studies have established CPET as a scientifically sound and, therefore, clinically valuable method for accurately assessing exercise limitation and prognosis in various disease states. The CPET is relatively easy to perform, yet complex in the interpretation of the results. The current review presents the principles of the analytic process of the test results, in order to obtain possible diagnoses.