Bronchopulmonary dysplasia (BPD) continues to be one of the most common long-term complications associated with preterm birth. Its incidence is increasing as the survival of extreme premature infants improves, but its clinical presentation is milder than the original description of Northway and collaborators. In contrast to the classic BPD that was strongly related to mechanical injury and oxygen toxicity, current forms of the condition are more related to immaturity, perinatal infection and inflammation, persistent ductus arteriosus and disrupted alveolar and capillary development. Many different definitions of BPD have been proposed, most of which are based on the duration of supplemental oxygen requirement. The different definitions can produce strikingly different incidence figures, which may account for the wide variations in the condition reported in the literature. Some of the limitations of the criteria most commonly used to diagnose BPD are discussed in this article.