The four previous articles in this series have traced the history of patient autonomy and have identified its ethical and legal foundations. Patient autonomy is highly valued in the United States to the extent that the patient does not lose the right of self-determination when he or she loses the capacity to make health-care decisions for him or herself. The law has devised several tools to promote "prospective autonomy." One mechanism is the instructional advance directive or living will. But most of us do not write such directives. Another mechanism is the proxy directive or durable power of attorney for health care, designating another person, a surrogate, to direct the course of our medical treatment upon our incapacity. But most of us do not do that either. Therefore, the most common mechanism by which our prospective autonomy is protected and promoted is through the informal selection of surrogates based on statutory priority lists. These "default" surrogates are the most numerous type of surrogate. This article explains the importance and legal fundamentals of surrogate decision making. It first describes five basic types of surrogates. The article then looks at the role of these surrogates and how they are supposed to make decisions on the patient's behalf. Unfortunately, surrogate performance is often mediocre or poor. There are significant and persistent obstacles to good surrogate decision making. After explaining these problems, the article concludes by identifying several solutions.