Classifications of interstitial (diffuse) lung disease in adults and children have undergone significant revision in recent years, with advances in our understanding of new entities and the biology and prognostic significance of certain histologic patterns. The contributions of the European Respiratory Society Task Force on Interstitial Lung Disease in Children and the North American Children’s Interstitial Lung Disease Group are reviewed, and a clinicopathologic classification of paediatric diffuse lung disease is summarized. Clinical characteristics and histologic definitions are also presented for selected entities within this classification, specifically, acinar dysgenesis, congenital alveolar dysplasia, alveolar capillary dysplasia with misalignment of pulmonary veins, abnormalities of alveolar growth, pulmonary interstitial glycogenosis, neuroendocrine cell hyperplasia of infancy, surfactant dysfunction disorders, obliterative bronchiolitis, hypersensitivity pneumonitis, and immunologic disorders. More uniform application of this diagnostic terminology in the future will allow more meaningful comparisons of different patient populations, radiologic-pathologic correlation, and development of disease-specific therapeutic strategies.