Routine vaccination of young puppies and kittens takes place within the first 16 weeks of life, during which time there is considerable change in the immune system of these animals. Newborn pups and kittens must obtain passive immune protection through the ingestion of colostrum within the first hours of life. The timing of early life vaccination is determined by the period of time required for passively acquired immunoglobulin to degrade, thereby permitting an endogenous immune response to be generated by the neonate. In the absence of inhibitory maternally derived antibody (MDA), pups and kittens are capable of mounting a protective immune response at an early age. New generation molecular vaccines appear able to circumvent the inhibitory effects of MDA. In addition to changes in serum immunoglobulin concentrations, there are alterations in the numbers and proportions of blood and tissue leucocytes (particularly CD4(+) and CD8(+) Tcells, and B cells) during the first year of life. The qualitative nature of the newborn immune system may also alter from Th2 regulation in utero to Th1 regulation in the neonatal period. Immune function is likely to be genetically determined, and in dogs there is evidence for breed effects on immune function which likely relate to the inheritance of particular haplotypes of major histocompatibility complex (MHC) genes. The design of vaccines for young animals of these species must take into account these immunological changes and the potential modulatory effect ofvaccines on immune development. (c) 2007 Elsevier Ltd. All rights reserved.