Pulmonary function tests (PFTs) are the conglomeration of series of assessments which are extremely beneficial in clinical evaluation of the respiratory system. Advancement of computer technology has given us the portable and automated devices making them a handy device in assessing the lung function. The purpose of this chapter is to discuss briefly the various tests and their use in distinguishing the types of pulmonary insufficiency and most importantly, their clinical applications and limitations. Pulmonary function tests have been indicated for the diagnosis of different types of pulmonary diseases and to assess effectiveness of therapeutic intervention. It helps the clinician to follow response to therapy, determine further treatment goals and in evaluating degree of disability. The gas exchange functions are assessed by measuring Alveolar-arterial PO2 gradient, diffusion capacity and gas distribution tests. Nowadays FeNO is being used as a biomarker for the assessment of airway inflammation and on comparison with conventional tests such as Bronchial provocation test (BPT) or the reversibility of FEV1, measurement of FeNO offer more advantages.