The original model of gene therapy, that of efficient delivery, durable transfer, and stable expression of transgenes to correct a gene defect underlying an inherited disease, is limited in light of improved understanding of the processes involved. Techniques that enable regulated expression of transgenes may enhance safety and allow us to regulate the timing and level of expression with a goal of precisely targeting a therapeutic level between the extremes of suboptimal and supraoptimal thresholds. Using regulated systems to control protein expression has practical and possibly essential roles for the success of safe and effective gene therapy in a number of clinical situations. Pharmacologically regulated gene expression is an evolving tool, and no individual system may be effective in all clinical applications.