Over the last decade, research in somatic gene therapy has focused on selected approaches to deliver therapeutic genes to cells both ex vivo and in vivo. While most current gene therapy clinical trials are based on cell-and viral-mediated approaches, nonviral gene medicines are emerging as potentially safe and effective in the treatment of a wide variety of genetic and acquired diseases. Nonviral technologies consist of plasmid-based expression systems containing a gene encoding a therapeutic protein and synthetic gene delivery systems. In addition to the therapeutic gene, plasmid-based expression systems contain other genetic sequences to control the in vivo production and secretion of a protein. They may include elements that prolong extrachromosomal gene expression, cell-specific promoters and, optionally, gene switches for enabling drug-regulated gene therapy. Unique gene delivery systems will be required depending upon the biology and (patho) physiology of the target tissue. This review provides a critical view of gene therapy with a major focus on advanced nonviral technologies to control the in vivo location and function of administered genes.