The interstitial cells in the kidney are not a homogeneous cell population but consist of different cell types like fibroblasts, dendritic cells or lymphocyte-like cells. Fibroblasts are the most abundant interstitial cell type. They are regarded as the most important cells for the production and degradation of extracellular matrix and are assumed to play a pivotal role in renal interstitial fibrosis, which correlates directly with the decrease in excretory renal function. Renal fibroblasts also have endocrine activity: cortical fibroblasts are supposed to synthesize erythropoetin, and inner medullary fibroblasts are involved in the regulation of water and electrolyte homeostasis. A powerful tool for the further elucidation of fibroblast function are studies on cultured cells. Different techniques for the isolation of fibroblasts have been reported, including the cultivation of fibroblasts from outgrowths of minced tissue and the selective removal of contaminating epithelial cells by various methods. Several aspects have to be considered while culturing fibroblasts. Fibroblasts in culture exhibit distinct morphologic and biochemical features depending on their site of origin, state of differentiation and culture conditions. Their identification in culture exclusively by morphological criteria is therefore critical especially in mixed cultures with other cell types. Unfortunately, a constitutively expressed, specific marker for all fibroblasts is still not available. Since myofibroblast formation is considered as a key event in renal interstitial fibrosis, the transformation of fibroblasts to myofibroblasts is of special interest. Studies on cultured fibroblasts provide an effective tool to examine factors that affect this transformation and regulate the production and degradation of extracellular matrix. In addition, this technique can be used for further characterization of the endocrine activity of cultured fibroblasts. A better understanding of the biology of fibroblasts is essential to develop therapeutic strategies for the treatment of renal tubulointerstitial fibrosis, the pathologic equivalent of progressive renal failure.