The search for methods of vaccine delivery not requiring a needle and syringe has been accelerated by recent concerns regarding pandemic disease, bioterrorism, and disease eradication campaigns. Needle-free vaccine delivery could aid in these mass vaccinations by increasing ease and speed of delivery, and by offering improved safety and compliance, decreasing costs, and reducing pain associated with vaccinations. In this article, we summarize the rationale for delivery of needle-free vaccines and discuss several methods currently in use and under development, focusing on needle-free injection devices, transcutaneous immunization, and mucosal immunization. Jet injectors are needle-free devices that deliver liquid vaccine through a nozzle orifice and penetrate the skin with a high-speed narrow stream. They generate improved or equivalent immune responses compared with needle and syringe. Powder injection, a form of jet injection using vaccines in powder form, may obviate the need for the "cold chain." Transcutaneous immunization involves applying vaccine antigen and adjuvant to the skin, using a patch or "microneedles," and can induce both systemic and mucosal immunity. Mucosal immunization has thus far been focused on oral, nasal, and aerosol vaccines. Promising newer technologies in oral vaccination include using attenuated bacteria as vectors and transgenic plant "edible" vaccines. Improved knowledge regarding the immune system and its responses to vaccination continues to inform vaccine technologies for needle-free vaccine delivery.