State laws govern the viability of causes of action for medical malpractice. The laws vary in terms of time limits to bring suit, qualifications of "expert" witnesses, cognizable theories of liability, and proper party defendants/proper party plaintiffs. Notwithstanding these differences, there are common requisites for all cases.

First and foremost, a physician must owe a duty to patients before his or her competency in performing that duty can be judged. In U. S. jurisprudence, a person has no affirmative duty to assist injured individuals in the absence of a special relationship with them (such as doctor-patient, attorney-client, guardian-ward, etc.). A doctor dining in a restaurant has no duty to come forward and assist a fellow customer who is suffering a heart attack. If the doctor merely continues with his meal and does nothing to help, the ailing person would not have an action for malpractice against him, not-withstanding their harm. However, once a doctor voluntarily decides to assist others or come to their aid, he or she becomes liable for any injury that results from any negligence during that assistance.

Once the requisite doctor-patient relationship is established, the doctor owes to the patient the duty to render care and treatment with that degree of skill, care, and diligence as possessed by or expected of a reasonably competent physician under the same or similar circumstances. The "circumstances" include the area of medicine in which the physician practices, the customary or accepted practices of other physicians in the area (the "locality rule"), the level of equipment and facilities available at the time and in that locality, and the exigent circumstances, if any, surrounding the treatment or medical service rendered. The requisite degree of skill and expertise under the circumstances is established by "expert testimony" from other practicing physicians who share the same or similar skill, training, certification, and experience as the allegedly negligent physician.

Vicarious Liability

Finally, a doctor who has been negligent may not be the only defendant in a subsequent lawsuit. A hospital that has retained the doctor on its staff may be vicariously liable for the doctor's negligence under a theory of "respondeat superior" ("let the master answer") that often holds an employer liable for the negligence of its employees. More often, the doctor has "staff privileges" at the hospital, and the hospital will attempt to prove the limited role it plays in directing or supervising the doctor's work. Importantly, many doctors belong to private medical practices, such as limited partnerships or limited liability companies, that also may be vicariously liable for the negligence of their member doctors.

However, a doctor is generally liable for any negligence on the part of his assistants and staff in carrying out his orders or caring for his patients. Likewise, an attending physician is generally liable for any negligence on the part of interns and medical students under the physician's guidance.