Pulmonary function testing is useful for the diagnosis and management of a variety of pulmonary conditions, and the most commonly obtained component is spirometry. Spirometry is most useful in the evaluation of obstructive airway disorders but can be a helpful tool in the management of patients with chest restriction or mixed disease. The utility of spirometry depends on reproducibility, standardization, and quality of testing. Accurate interpretation of test results depends on the availability of reference equations applicable to the subject undergoing testing. This paper reviews basic concepts, testing procedures, and interpretation of a single set of spirometry results as well as results obtained over time and gives an overview of previously published reference equations.