The use of modern diagnostic equipment to measure pulmonary function has reached a substantial level of maturity after many decades of evolution. The ability to differentiate a normal result from a measurement that is potentially indicative of disease requires the use of normal reference values that have similarly become increasingly robust with larger and more diverse populations. The use of reference equations should carefully consider the individual being tested in relation to the reference population and a detailed understanding of the sources of variation related to geographic, socioeconomic, environmental and genetic factors.