2016 by John Wiley & Sons, Inc. All rights reserved. Active immunization, or the use of antigens (formulated as a vaccine) to stimulate an immune response, is one of the best strategies to control and prevent infectious diseases in horses and other domestic animals. The types of vaccines licensed for use in horses are: attenuated or modified live vaccines (MLV); killed or inactivated vaccines; subunit vaccines; recombinant vectored vaccines; nucleic acid-based vaccines; and chimera vaccines. The immune response can be influenced by host factors, such as genetics, degree of existing immunity to the antigen (i.e., first time versus subsequent exposure), age, the immune status of the vaccinated animal, presence of maternal antibodies, and pregnancy status. Nanoparticles and microparticles are antigen delivery systems with dimensions similar to pathogens. Recently, it has been described that polymeric particles also have adjuvant properties, such as complementing the activity of TLR ligands and engaging NALP3 inflammasome.