Dealing with paediatric asthma in daily practice, we are mostly interested in the airway function: the hallmark of asthma is the variability of airway patency. Various pulmonary function tests (PFT) can be used to quantify airway caliber in asthmatic children. The choice of the test is based on the developmental age of the child, knowledge of the diagnosis/underlying pathophysiology, clinical questions and reasoning, and treatment. PFT is performed to monitor the severity of asthma and the response to therapy, but can also be used as a diagnostic tool, and to study growth and development of the lungs and airways. This review aims to provide clinicians an overview of the differences in assessing PFT in infants and preschool children compared with older cooperative children, which tests are feasible in infants and young children, the limitations of and usefulness of these tests, and of their interpretation in these age groups.