Undue influence (UI) is a psychological process by which a person's free will and judgment is supplanted by that of another. It is a legal term and the strict definition varies by jurisdiction. Generally speaking, it is a means by which a person gains control over their victims' decision making through manipulation tactics and unfair pressure, typically for financial gain. Historically, UI has been poorly understood, even in some legal circles.

Undue influence is typically perpetrated by a person who is trusted by the victim and is dependent on them for emotional and physical needs. Caregivers are often found to have unduly influenced their patients, however, anyone in a position of trust and authority over the victim (for example, fiduciary) may be guilty. This includes the victims' attorney, accountant, nursing home attendant, or even children. UI is a process, not a single event. A manipulator may spend weeks, months, or even years before successfully unduly-influencing their victim. Anyone is susceptible to UI, but the elderly are particularly vulnerable. Perpetrators of UI operate in shadow, enacting their manipulation behind closed doors where there are no witnesses. A person being unduly influenced may become withdrawn, depressive and passive. Physical signs of abuse may include unexplained bruising, scratches, and or broken bones. Other signs may include missing or broken dentures, prescription eyeglasses, hearing aids or other assistive devices.

A distinction is made between the nature of capacity and undue influence. In assessing capacity, the practitioner evaluates an individual's ability to competently perform tasks (for example, execute a will or give medical consent). These assessments give insight to the functioning of the cognitive capabilities at that moment in time. Conversely, screening for undue influence is focused on the process of events which occur over an extended period. To determine whether another person is leveraging unfair tactics on the victim, an assessment specific to undue influence is required.

Undue influence occurs in various circumstances including, but not limited to domestic violence, hostage situations, cults, prisoners of war, and dictatorships. The common theme among these situations is the aspect of psychological manipulation. Traumatic bonding may occur between the victim and influencer, as a result, the victim may even defend the perpetrator. The effectiveness of cult tactics (for example, Love bombing) on young and healthy individuals illustrates that anyone, regardless of mental status, is a potential victim of UI under certain circumstances.

Elderly Americans are living longer, and with this increased life expectancy, the prevalence of cognitive disorders associated with advanced age has also increased. A significant concentration of wealth is controlled by this aging demographic making them a potential target for exploitation. Modern families are becoming more complex and dispersed. Given these factors, the number of will contests involving undue influence is expected to increase.