Numerous chemical, physical, and viral methods have been developed for delivering genes to mammalian cells in culture. Physical methods of gene transfer have the advantage of transporting the genetic material directly into the cytoplasm or nucleus of the cell, bypassing the need for specific membrane requirements for transport into the cell. Several different instrument designs have been described for biolistic delivery of genes (1 -5 ), however, only two of these, the PDS-1000/He and the Helios Gene Gun, are commercially available. Although both instruments may be used for delivery of DNA into mammalian cells in vitro, in this chapter I describe protocols using the PDS-1000/He. In experiments comparing gene expression in cells bombarded using both instruments, expression is routinely two- to three-fold higher in cells bombarded using the PDS-1000/He (unpublished results).