Recent advances in recombinant DNA technology have permitted the direct cloning of DNA fragments (either derived from naturally occurring or artificially designed gene sequences) into various cloning vectors including bacteriophages, plasmids, and viruses. Such recombinant constructs represent the basic reagents of molecular biology. A major application utilizing cloned DNA sequences is the expression of the cloned DNA into a protein product, i.e., the expression of recombinant genes. Because the cloned DNA sequences may be modified or altered, recombinant expression technology thus enables the investigator to “custom-design” the final protein product. Furthermore, most expression vectors are designed to allow the linking of various “tags” to the expressed recombinant protein to facilitate subsequent detection and isolation. This chapter provides a brief overview of the technologies currently employed in “tagging” expressed recombinant proteins and the corresponding detection and isolation methodologies, as well as some of the applications utilizing recombinant gene products.