The molecular cloning of the cDNA sequences encoding most G-protein-coupled receptors, including those from humans, allows their study in a variety of recombinant systems. In this respect, transfected mammalian cell lines constitute the most frequently used model for investigating the pharmacological and biochemical properties of these receptors. Several protocols have been described (based on the use of calcium phosphate precipitation, DEAE dextran, cationic lipids, and electroporation), allowing their transient or stable expression in diverse cell lines. This chapter gives a brief overview of the different techniques and provides methodology for the generation of transiently transfected cells and for selection, isolation and maintenance of stable transfected cell lines.