The most abundant cell type in the lung interstitium is the fibroblast. These fibroblasts resemble ordinary fibroblasts, but also possess some distinguishing features such as long, branching processes and gap junctions. Pulmonary fibroblasts function to produce type III collagen, elastin, and proteoglycans of the extracellular matrix of the alveolar septa. They play an important role in the repair and remodeling processes following tissue damage. The controlled accumulation of fibroblasts to sites of inflammation is crucial to effective tissue repair after injury. An inadequate or an excessive accumulation of fibroblasts can result in abnormal tissue function. For example, excess proliferation of fibroblasts contributes to adventitial thickening observed during the development of hypoxia-induced pulmonary hypertension. Rat Pulmonary Fibroblasts provide an invaluable tool for understanding physiological and pathophysiological relevance in lung function and disease.