Vaccines are materials which when introduced into the human body help protect the vaccinated person against specified communicable diseases. Vaccines are preparations of dead or weakened pathogens or their products that, when introduced into the body, stimulate the production of protective antibodies or T-cells without causing the disease. Vaccination is also called active immunization because the immune system of the body is stimulated to actively develop its own immunity against the pathogen. In contrast to that, passive immunity results from the injection of antibodies formed by another animal (e.g. horse, human) which provide immediate, but temporary, protection for the recipient. The greatest triumph of vaccination is the eradication of smallpox from Earth; no naturally-occurring cases have been reported since 1977. Due to the success of vaccination, nearly 100% reduction has been obtained in the cases of many diseases which were previously sources of great mortality and morbidity. These include diphtheria, measles, mumps, pertussis, rubella, and tetanus. This chapter discusses nature and importance of vaccines, traditional and modern methods of vaccine production, newer approaches in vaccinology, production of virus vaccines, production of bacterial toxoids, production of killed bacterial vaccines, and vaccine production versus other aspects of industrial microbiology.