This paper discusses the efficacy of cat and dog vaccines, with respect to duration of immunity and regulatory issues. The European Union (EU) regulatory requirements are described: briefly, efficacy claims, which include duration of immunity, have to be specific for the product and supported by controlled laboratory and field trials. As a result, the duration of immunity shown has typically been a minimum, because of the cost and welfare implications of keeping animals for long periods of time in isolation. In contrast, in the US, duration of immunity has not traditionally been required for each individual product, only for rabies vaccines and some other vaccines for which no other products are available. The consequence of this is that in the US, various scientific authorities have produced guidelines appropriate for individual diseases. Undoubtedly this will continue, although the regulatory authorities also appear to be moving towards a position where lack of information may be indicated on the product label, and studies are required to support extended duration of immunity claims. The advantages and disadvantages of laboratory challenge studies versus field trials are discussed, and the use of alternatives such as surrogate markers of protection. The approaches used for small animal vaccines are compared to those used, for example, in human medicine. The main issue for small animals is that unlike in some other species, the aim generally is to maximise protection in the individual, rather than induce protection at the population level. The drawbacks of the present EU system are summarised, and the ways in which the situation is currently being approached and improved are presented.