Pulmonary function tests permit an accurate assessment of the functional state of respiratory system with quantification of the severity of the disease, if any. Particularly these tests provide important information relating to the process of ventilation and diffusion of gases across the alveolar membrane. The construal of pulmonary functions tests require concept of respiratory physiology as the ventilation is dependent to a large extent on the activity of respiratory muscle especially diaphragm and external intercostal muscle in relation to intra pulmonary and intrapleural pressures, lung compliance, alveolar surface tension and large and small airways resistance. Diffusion of gases across the alveolar membrane is principally determined by the surface area, integrity of the alveolar membrane and the pulmonary vascular bed. However, to conclude the respiratory status of any person the patient’s clinical history, presentation and other basic investigations are also imperative. Although, profound amount of in depth information is available on all the aspects of PFTs, this review will be very helpful to understand these tests in a simple way.