Interstitial lung diseases are a mixed group of diffuse parenchymal lung diseases which can have an acute or chronic course. Idiopathic diseases and diseases with an underlying cause (e.g. collagen vascular diseases) share the same patterns. Thin section computed tomography (CT) plays a central role in the diagnostic work-up. The article describes the most important interstitial lung diseases following a four pattern approach with a predominant nodular or reticular pattern or a pattern with increased or decreased lung density.