Usual interstitial pneumonia (UIP) is a distinct histological entity that is observed in idiopathic pulmonary fibrosis (IPF). The histopathological hallmark of UIP is a discontinuous, patchy and temporally heterogeneous interstitial fibrosis driven by aggregates of myofibroblasts, so-called fibroblastic foci with secondary honeycomb changes and smooth muscle proliferation (Figure 1). Typically, fibroblastic foci appear as conglomerates localized in the pulmonary interstitium beneath the alveolar epithelium and in the bronchi, with the long axis of the spindle-shaped myofibroblasts running parallel to the epithelial lining. Although fibroblast foci are not specific for UIP, their presence in the right clinical and pathological setting strongly supports the diagnosis. UIP is the most common of the idiopathic interstitial pneumonias (IIP) and a progressive, usually fatal disease that shows little, if any, response to therapy. Therefore, it is critically important i. to separate UIP from other histologic subsets of IIP and ii. from other interstitial lung diseases that have a more favourable response to a specific therapy and better prognosis.