Cancer is the uncontrolled growth of abnormal cells. The speed with which a cancer spreads and the severity of the disease it causes depends on the type of tissue cell affected. As many as one in five dogs are likely to develop one of the many different forms of cancer at some stage of their lives. The risk of developing cancer increases with age. This means that, as dogs now enjoy a longer life expectancy through improved veterinary care, the number of animals with cancer has been increasing in recent years.