To prevent unwanted litters, to prevent undesirable behaviour and bodily changes (aggression, possessiveness, restlessness, escaping from properties, bleeding from vulva, roaming for mates etc.). Equally importantly, to prevent cancer in later life (the older an unsterilised animal, the greater their chances of developing a range of different cancers related to the reproductive tract) and in females to prevent a potentially life threatening condition known as pyometra (an infection of the uterus that occurs when the cervix in a dog that has not been spayed shuts and traps material in the uterus which then starts putrefying).