Respiratory disorders are commonly classified into three basic categories: Obstructive disorders, Restrictive disorders and Mixed disorders. Pulmonary function tests distinguish between these three categories of respiratory disorders. Spirometry is the basic test of pulmonary function which is commonly used for diagnosis, assessment and reassessment purposes. All the lung volumes and capacities can be measured with the help of the spirometer, except for Functional residual capacity (FRC), Residual volume (RV) and Total lung capacity (TLC). FRC, RV and TLC are measured by other three methods, namely, Helium dilution method, Nitrogen wash out method and Body plethysmography. Results obtained from the pulmonary function tests should be interpreted accurately and the physical therapy protocol should be established accordingly.