Cardiopulmonary exercise testing (CPET) is a procedure widely used in daily clinical activity to investigate cardiac and pulmonary disorders. Peak oxygen consumption (VO2 peak) is the most validated and clinically accepted parameter used to report aerobic capacity in healthy individuals and in different clinical settings. However, peak VO2 is influenced by several factors, whose variability is nowadays particularly evident due to the extensive use of CPET even in very young and very old subgroups of patients. Thus, its diagnostic and prognostic significance may be improved by the use of % of predicted VO2. At present, many sets of normal values are available, making the identification of the most proper max VO2 predicted value a challenging problem. In fact, normal value sets have been obtained from studies whose accuracy was reduced by important limitations, such as small sample size, low grade of heterogeneity of the population enrolled, poor rigorousness of methods, or difficulty in interpreting results. Accordingly, the aim of the present review is threefold: (A) to report some illustrative cases to show how the choice of the normal value set can influence the report of CPET; (B) to describe the most known and used reference value sets, highlighting the main characteristics of sample population, the most important methodological aspects, and the major limitations of the studies; (C) to suggest which equation should be used, if any, and to underline its weakness.