The intent of this chapter is to familiarize the reader with the physiologic principles underlying the interpretation of CPET (comprehensive cardiopulmonary exercise test) results. This chapter presents a simple algorithm for interpreting CPET test results in subjects referred for the diagnosis of unexplained dyspnea. To illustrate, data from a patient with unexplained dyspnea is presented and interpreted. In summary, the CPET result allows the clinician to assign dyspneic subjects to one of three categories: normal function, subnormal function but no organic illness, or organic illness of the cardiac, pulmonary, or skeletal muscle systems. To arrive at a specific diagnosis, further evaluation is necessary.