Self-harm is intentional behavior that is considered harmful to oneself. This is most commonly regarded as direct injury of one's own skin tissues usually without a suicidal intention. Other terms such as cutting, self-injury, and self-mutilation have been used for any self-harming behavior regardless of suicidal intent. The most common form of self-harm is using a sharp object to cut the skin. Other forms include scratching, hitting, or burning body parts. While earlier usage included interfering with wound healing, excessive skin-picking, hair-pulling, and the ingestion of toxins, current usage distinguishes these behaviors from self-harm. Likewise, tissue damage from drug abuse or eating disorders is not considered self-harm because it is ordinarily an unintended side-effect but context may be needed as intent for such acts varies.

Although self-harm is by definition non-suicidal, it may still be life-threatening. People who do self-harm are more likely to die by suicide, and self-harm is found in 40–60% of suicides. Still, only a minority of those who self-harm are suicidal.

The desire to self-harm is a common symptom of some personality disorders. People with other mental disorders may also self-harm, including those with depression, anxiety disorders, substance abuse, mood disorders, eating disorders, post-traumatic stress disorder, schizophrenia, dissociative disorders, and gender dysphoria. Studies also provide strong support for a self-punishment function, and modest evidence for anti-dissociation, interpersonal-influence, anti-suicide, sensation-seeking, and interpersonal boundaries functions. Self-harm can also occur in high-functioning individuals who have no underlying mental health diagnosis. The motivations for self-harm vary. Some use it as a coping mechanism to provide temporary relief of intense feelings such as anxiety, depression, stress, emotional numbness, or a sense of failure. Self-harm is often associated with a history of trauma, including emotional and sexual abuse. There are a number of different methods that can be used to treat self-harm, which concentrate on either treating the underlying causes or on treating the behavior itself. Other approaches involve avoidance techniques, which focus on keeping the individual occupied with other activities, or replacing the act of self-harm with safer methods that do not lead to permanent damage.

Self-harm is most common between the ages of 12 and 24. Self-harm in childhood is relatively rare, but the rate has been increasing since the 1980s. Self-harm can also occur in the elderly population. The risk of serious injury and suicide is higher in older people who self-harm. Captive animals, such as birds and monkeys, are also known to participate in self-harming behavior.