The changes in clinical presentation of bronchopulmonary dysplasia (BPD) in recent years have made many of the original definitions of BPD obsolete. The use of supplemental oxygen as a criterion for BPD diagnosis has many limitations. Supplemental oxygen is necessary to treat these infants, but at the same time it plays an important role in the pathogenesis of BPD. Because there are no accepted standards for supplemental oxygen administration, there are wide variations for its indications among different centers and this has a marked effect on the reported incidence of BPD. For this reason, it is essential to standardize the indications for supplemental oxygen when duration of oxygen therapy is used as the main criteria to diagnose BPD. Using supplemental oxygen need at specific time points does not necessarily reflect chronic lung damage and should be avoided as a single diagnostic criterion for BPD. A prolonged duration of supplemental oxygen is necessary to demonstrate the presence of chronic lung damage. The criteria based on supplemental oxygen at 36 weeks postmenstrual age has gained wide acceptance, but it is a less stringent criterion for the more mature infants. The longer the gestation, the shorter the time on oxygen that is required to meet this BPD criterion. None of the proposed criteria based on duration of oxygen therapy have shown a strong predictive value for long-term outcome. In view of all these shortcomings, it is essential to develop more objective physiologic tools to define the degree of lung damage and improve the prediction for long-term outcome in these infants.