Pulmonary functions tests have come to play a major role in Pediatric Pulmonology. These tests have started assuming considerable importance that it is now-a-days considered almost akin to taking blood pressure for diagnosing hypertension. Pulmonary function tests helps to differentiate important lung conditions and also helps to monitor the response to treatment thereby facilitating optimal management. The chapter provides insight into the various aspects of pulmonary function tests and their clinical utility.