Mental health is the successful performance of mental function, resulting in productive activities, fulfilling relationships with other people, and providing the ability to adapt to change and cope with adversity.  The term mental illness refers collectively to all diagnosable mental disorders—health conditions characterised by alterations in thinking, mood, or behaviour associated with distress or impaired functioning.   Mental health and mental illness are two continuous concepts. People with optimal mental health can also have mental illness, and people who have no mental illness can also have poor mental health.

Mental health problems may arise due to stress, loneliness, depression, anxiety, relationship problems, death of a loved one, suicidal thoughts, grief, addiction, ADHD, self-harm, various mood disorders, or other mental illnesses of varying degrees, as well as learning disabilities. Therapists, psychiatrists, psychologists, social workers, nurse practitioners, or family physicians can help manage mental illness with treatments such as therapy, counselling, or medication.