Improvements in veterinary healthcare over recent decades mean that we now have a significant population of geriatric small companion animals. The design of optimum nutritional and vaccination programmes for these aged animals must be underpinned by knowledge of the physiological changes that occur in later life. It is clear that older dogs and cats are affected by the process of immunosenescence and that similar changes occur in these species to those documented in elderly people. The most consistent findings of recent investigations indicate impairment of cell-mediated immune function with age. Senior dogs and cats are generally shown to have reduced blood CD4(+) T cells (with imbalance in Th1 versus Th2 functional activity), elevation in the CD8(+) subset and reduction in the CD4:CD8 ratio. The ability of blood lymphocytes to respond to stimulation by mitogens decreases, as does the cutaneous delayed type hypersensitivity response. By contrast, there is relative preservation of the ability to mount humoral immune responses. Serum and salivary immunoglobulin (Ig)A production increases and IgG concentration remains unaltered with age. Elderly animals generally have persisting vaccinal antibody titres at protective level and respond to booster vaccination with elevation in titre. Older dogs and cats are able to make primary humoral responses to novel antigens, but the magnitude of these may be reduced relative to titres achieved in younger animals. Fewer investigations have studied the phenomenon of 'inflammageing' (the effect of cumulative antigenic exposure and onset of late life inflammatory disease) in these species. Senior cats have increased production of pro-inflammatory cytokines by blood monocytes, but this effect has not been demonstrated with cells derived from older dogs. Numerous studies have investigated whether canine and feline immunosenescence might be slowed or reversed by dietary supplementation with antioxidants, but no significant research has addressed the need for geriatric vaccination protocols.