DNA transformation in bacteria has permitted detailed genetic analysis as well as the implementation of recombinant DNA technology. The utility of manipulation and cloning of genes in bacteria is well recognized. This chapter will concern the manipulation and cloning of genes in eukaryotic systems ranging from yeast to animal systems. Gene manipulation utilizing eukaryotic cloning systems permits the analysis of regulation of eukaryotic genes in their native environment and offers certain advantages for the production of foreign gene products which may require post-transcriptional modifications which are indigenous to eukaryotic organisms.