Desexing and neutering are umbrella terms for the surgical sterilisation of both male and female animals. Females are ‘speyed’ by removing the ovaries and uterus, and males are ‘castrated’ by removing the testicles. It has always been a convenient way to provide contraception and prevent many diseases, and it has traditionally been performed at the age of 6 months. However, our increasing knowledge of the effects of the surgery as well as the advent of other health issues as a result of breeding mean the question of when to desex and what else should be done at the same time is important. These considerations vary depending on the breed and the individual. The question of age and reproductive system diseases Speying a female dog and cat significantly reduce the incidence of mammary (breast) cancer and pyometra (infected uterus) later in life. Mammary cancer is extremely common in entire females or those desexed after a few heats, and even having one heat prior to desexing can increase the risk greatly. If a bitch has been used for breeding, it is also extremely worthwhile desexing her when her breeding life has finished as pyometra is a life-threatening and common illness. Castration of male dogs prevents testicular cancer and many prostatic diseases, including cancer. Once again, the later the procedure is done, the higher the risk of developing issues. Male cats are not at as much risk in this respect. As a rule, the younger the animal and less sexually mature at the time of desexing, the lower the reproductive disease risk. The question of age and breed-related diseases There has been some evidence that certain breeds of dogs are more prone to orthopaedic problems and some cancers and that their age at desexing may influence the incidence of these problems later. Every breed is completely different and so what applies to one breed must not be automatically applied to another. Scientific research has only just started into this issue and currently the only studies