The recent developments in modern vaccinology are mainly based on: (i) cloning of microbial genes into recombinant vectors containing genetic information for expression of desired neutralizing immunogens; (ii) alternatives of attenuated vectors with deleted genes permitting the insertion of several foreign genes expressing antigens exposed to the host immune system during the abortive replication of such vectors; (iii) combined vaccines with the aim to protect against many diseases with a limited number of administrations; (iv) evidence demonstrating the ability of animals to respond serologically to DNA injections considered as a potential method of vaccination; (v) the possibility to manipulate the immune system with new and improved immunomodulators enhancing the immune response; and (vi) new microcarrier systems for particular immunogens or immunomodulators delivery, either in a single dose or sustained release, and presentation to the immune system for a relevant response. New vaccines being developed are mainly based on viral, bacterial or other vectors modified with genetic engineering technology, to possess and express desired antigens for vaccination against single or multiple infections. Existing combined vaccines like diphtheria, tetanus, pertussis (DTP) are also experimented with new additional components like recombinant hepatitis B virus surface antigen, inactivated poliovirus, and Haemophilus inJluenzae type b immunogens, in order to produce multivalent vaccines. Such types of vaccines permitting the reduction of multiple medical visits is of particular interest to pediatric immuni zation programs, and would benefit especially the developing countries assuring better vaccine compliance with immunization schedules.