The term "Incapacitated Person" is used to refer to someone who does not have the legal capacity to make their own decisions.  An incapacitated person is defined as someone who "for reasons other than advanced age or minority, has a clinically diagnosed condition that results in an inability to receive and evaluate information or to make or communicate decisions to such an extent that the individual lacks the ability to meet essential requirements for physical health, safety, or self-care, even with appropriate technological assistance".

An intellectually disabled individual is defined as someone who has significantly sub average intellectual functioning (usually defined as an IQ of less than 70), and who has limitations in two or more areas of adaptive skills such as communication, self-care, social skills, health, and safety. Note: The term "Mental Retardation" has been removed from all Massachusetts statutes and replaced with "intellectual disability".  A guardian may be appointed for a person with an intellectual disability.

A guardian may be appointed for a person with mental illness.  Mental illness is defined as a medical condition that disrupts a person's thinking, feeling, mood, ability to relate to others, and daily functioning. Mental illness often results in a diminished capacity for coping with the ordinary demands of life. People suffering with a mental illness can experience relief from their symptoms by actively participating in a treatment plan.

A guardianship may be necessary when, for example, an elderly parent has a degenerative health condition and cannot therefore consent to treatment, or placement in a nursing facility, or, a person has suffered a traumatic brain injury, or, a disabled child is turning eighteen years old.

The person alleged to be incapacitated is called the Respondent.