<p>Fibrosis is a complex disorder characterized by the excessive deposition of extracellular matrix, which disrupts normal tissue architecture and compromises organ function. Fibrosis can affect any organ, with pulmonary fibrosis being one of the most common and life-threatening forms. Despite marked research efforts, effective antifibrotic therapies remain limited, largely due to an incomplete understanding of the underlying disease mechanisms. At the centre of fibrotic processes are fibroblasts, which are tissue-resident mesenchymal cells responsible for extracellular matrix production, tissue remodelling, wound healing and fibrosis. For decades, the biology of fibroblasts remained poorly understood, but advances in single-cell sequencing have recently provided deeper insights into their heterogeneity, plasticity and functional diversity. These insights have prompted renewed efforts to identify the core regulatory programmes that govern fibroblast states in health and disease. In this Review, we examine how immunological, mechanical and metabolic regulators influence fibroblast function in fibrosing interstitial lung diseases. We show how loss of stromal regulation through chronic inflammation, immune dysfunction, altered tissue biomechanics and metabolic stress can tip the balance from successful tissue repair to progressive fibrosis.</p>