There is strong evidence that people with dementia in care homes and hospital wards do not go outside and that, if there is outdoor space, it is not usually dementia friendly. This article reviews the evidence which shows that being outside is essential for mental and physical health and well-being. Vitamin D deficiency is due mainly to a lack of exposure to sunshine, and has been shown to be associated with falls and with a low mood and cognitive impairment. Physical exercise is also important for health and can reduce the risk of falls. Going outside and keeping active have been shown to: improve general health; reduce risk of depression; reduce cognitive decline; provide older people with a sense of freedom; improve sleeping patterns; improve appetite; reduce incontinence; and reduce aggressive behaviour. The article argues that the benefits of being outside, of being exposed to light, and taking part in exercise can lead to a reduction in the use of drugs. Savings in the cost of care can be used in the creation of therapeutic outdoor spaces. A second article will cover the key design features that can enable the use of outside space.