Picrosirius red (PSR, aka Sirius red) staining is a histochemical technique for the identification of collagen and its specificity makes it suitable for robust quantitative assessments of collagen in tissue sections. Such digital image analysis is particularly valuable for the evaluation of fibrosis and tissue remodelling. The PSR dye stains collagen fibres red, while also making them birefringent. There is some non-specific red staining of tissue components, which may include nuclei, keratohyalin granules and mucus, however these are not rendered birefringent so do not impact quantification using polarised light microscopy. The non-specific nuclear staining can be prevented by completing the optional heteropolyacid pre-treatment step in this protocol. Some authors use PSR to distinguish between type I and type III collagen fibres, but recent analysis has shown that this is not always reliable, with immunohistochemistry providing a more definitive distinction.