Pets, or companion animals, are said to be good for people. Until recently there has been little serious study of the effects on people's health of their interactions with companion animals. This is in spite of the fact that they have shared human lives for centuries and their beneficial effects have been known for at least 200 years. This paper reviews the ways in which companion animals have favourable effects on human health and behaviour, for example, as guides for blind and deaf people, for enriching the lives of long stay patients and for providing physical activity like horse riding for the severely disabled. Current knowledge of the effects of animals on human psychological, behavioural, physiological and social development is reviewed, including the use of animals in prison programmes. New findings in Australia show that pet owners had marked reduction in risk factors related to cardiac disease compared with non-owners. Other recent work has indicated that companion animals are able to act as 'early warning systems' for acute human conditions such as epileptic seizures.