Immortalization of chondrocytes increases life span and proliferative capacity but does not necessarily stabilize the differentiated phenotype. Expansion of chondrocyte cell lines in continuous monolayer culture may result in the loss of phenotype, particularly if high cell density is not maintained. This chapter describes strategies for maintaining or restoring differentiated phenotype in established chondrocyte cell lines involving culture in serum-free defined culture medium, in suspension over agarose or polyHEMA, or within alginate or collagen scaffolds. Chondrocyte cell lines have been used successfully to develop reproducible models for studying the regulation of gene expression in experiments requiring large numbers of cells. Thus, approaches for studying transcriptional regulation by transfection of promoter-driven reporter genes and cotransfection of expression vectors for wild-type or mutant proteins are also described.