The recent publication of guidelines for the control of reproduction in dogs and cats shows that a variety of non-neoplastic and neoplastic conditions occur more commonly in spayed/neutered than entire dogs, and for several of such conditions the earlier the age at surgery - the higher the risk. In addition, unwanted behavioural changes may occur after gonadectomy rather than expected improvement in behaviour in some dogs. However, the fact that surgical gonadectomy increases the risk of any such condition does not mean that dogs should not be spayed/neutered any more. Gonadectomy still plays a major role in lowering the risk of mammary neoplasia and pyometra in bitches and of prostatic hypertrophy in male dogs. The evidence that gonadectomy influences the development of a condition is often conflicting, and it varies depending on the breed, sex and condition. This paper briefly illustrates current knowledge on the incidence of the above conditions in gonadectomised dogs, providing information on whether or not - and what is the best age at which - to perform gonadectomy in our companion dogs based on available scientific data.