Key pointsPresentation of ILD is very non-specific.Imaging, especially HRCT, confirms the presence of ILD, but is rarely diagnostic.Invasive diagnosis is almost always needed.Bronchoscopy rarely gives diagnostic information in paediatric ILD.For most paediatric ILD, transbronchial or percutaneous lung biopsy gives insufficient tissue for diagnosis and carries increased risk.Open lung biopsy or VATS is the diagnostic investigation of choice.Educational aimsTo allow the reader to correctly identify infants and children with interstitial lung disease (ILD).To help the reader to appreciate the wide differential diagnosis, including that of disorders specific to infancy, comprising the surfactant protein disorders.To explain the strengths and weaknesses of the different investigative modalities used in establishing a firm diagnosis.To discuss the limited evidence for the different treatment options currently in use.SummaryPaediatric interstitial disease is rare, and comprises many disparate diseases. This article aims to provide the reader with an update on how to recognise infants and children who should be considered as possibly having ILD. The diagnostic process, including the roles of HRCT and bronchoscopy, and the different methods of lung biopsy, will be set out. The newly recognised entities, such as neuroendocrine cell hyperplasia of infancy, pulmonary interstitial glycogenosis and the disorders of surfactant protein metabolism, are also described. Finally, the article reviews the evidence that exists to guide treatment of these conditions.