Nonviral gene transfer is markedly enhanced by the application of in vivo electroporation (also denoted electrogene transfer or electrokinetic enhancement). This approach is safe and can be used to deliver nucleic acid fragments, oligonucleotides, siRNA, and plasmids to a wide variety of tissues, such as skeletal muscle, skin and liver. In this review, we address the principles of electroporation and demonstrate its effectiveness in disease models. Electroporation has been shown to be equally applicable to small and large animals (rodents, dogs, pigs, other farm animals and primates), and this addresses one of the major problems in gene therapy, that of scalability to humans. Gene transfer can be optimized and tissue injury minimized by the selection of appropriate electrical parameters. We and others have applied this approach in preclinical autoimmune and/or inflammatory diseases to deliver either cytokines, anti-inflammatory agents or immunoregulatory molecules. Electroporation is also effective for the intratumoral delivery of therapeutic vectors. It strongly boost DNA vaccination against infectious agents (e.g., hepatitis B virus, human immunodeficiency virus-1) or tumor antigens (e.g., HER-2/neu, carcinoembryonic antigen). In addition, we found that electroporation-enhanced DNA vaccination against islet-cell antigens ameliorated autoimmune diabetes. One of the most likely future applications, however, may be in intramuscular gene transfer for systemic delivery of either endocrine hormones (e.g., growth hormone releasing hormone and leptin), hematopoietic factors (e.g., erythropoietin, GM-CSF), antibodies, enzymes, or numerous other protein drugs. In vivo electroporation has been performed in humans, and it seems likely it could be applied clinically for nonviral gene therapy.