Most mental health problems are not easily defined. The American Diagnostic and Statistical Manual of Mental Disorders and the International Statistical Classification of Diseases and Related Health Problems are most generally used.

Estimates to the prevalence of mental illnesses can vary significantly, depending on how the question is presented. The 2014 Adult Psychiatric Morbidity Survey found that 1 in 6 respondents had shown the symptoms of a common mental disorder in recent days, and 1 in 8 reported seeing mental health treatment. In the same year, the Health Survey for England found that 25% of respondents had been diagnosed with a mental illness at some point in their life and a further 18% had had one that was not diagnosed. A survey in Scotland found 26% of respondents reported having experienced a mental health problem at some point in their life, but the figure increased if respondents were shown a list of conditions. A 2017 survey found that 65% of Britons have experienced a mental health problem, with 26% having had a panic attack and 42% saying they had suffered from depression.

Surveys have found that mental health problems have been on the rise since 2000, although growing awareness may also be a factor, and there are some countertrends such as a decline in suicide. One survey found that the number of responders who had reported having suicidal thoughts in the past year increased from 3.8 per cent in 2000 to 5.4 per cent in 2014. 2018 was the first year that mental health factors like stress and anxiety caused over half of all absences from work. According to a survey of 3,500 participants by the Office for National Statistics (ONS), the number of adults in Britain with depression has doubled during the coronavirus pandemic with 19.2% experiencing depression in June 2020. | Wikipedia