Cardiopulmonary exercise testing (CPET) is important for the differential diagnosis of dyspnoea-fatigue syndromes. The test more typically includes measurements of ventilation (VE), carbon dioxide output (VCO2), and oxygen uptake (VO2) at a progressively increased workload (W) until a maximum VO2, called VO2max or VO2peak to define aerobic exercise capacity, but steady state evaluations have utility in some contexts. The difference in VO2max and VO2peak can sometimes be made by the identification or not of a VO2 plateau while workload may be still increasing. However, the information content of VO2max and VO2peak is essentially the same, provided other criteria of maximum exercise are met, with, importantly, a respiratory exchange ratio (RER) ≥1.1. Ventilatory reserve may be calculated at VO2peak. This is the difference between maximum voluntary ventilation (MVV) and VEpeak, with MVV either directly measured, or predicted from the forced expiratory volume in 1 sec (FEV1) times 35 or 40. The ventilatory reserve normally ranges from 20 to 60 L/min, with an extreme lower limit of normal of 11 L/min, although caution should be applied to this measure given the unreliability of calculated MVV. Other relevant CPET measurements are VO2 at the anaerobic threshold (VO2AT) (RER = 1), VE/VCO2 either as a slope over the entire CPET or, preferably, at the AT, maximum heart rate and recovery, and the VO2-work rate relationship (ΔVO2/ΔW). Indeed VE/VCO2 slope has been shown to be a powerful prognostic indicator in heart failure independent of peak VO2.