This article proposes a model of medical decisions based on 2 fundamental characteristics of each decision--importance and certainty. Importance reflects a combination of objective and subjective factors; certainty is present if 1 intervention is superior and absent if 2 or more interventions are approximately equal. The proposed model uses these characteristics to predict who will have decisional priority for any given decision and shows how one class of decisions lends itself particularly well to shared decision making. Three other types of decisions are less well suited to a collaborative decision: 1) For major choices that have low certainty, patients should be encouraged to be the primary decision makers, with physician assistance as needed. 2) Most minor decisions that have high certainty are expected to be made by physicians. 3) Major decisions that have high certainty are likely to cause serious conflict when patients and physicians disagree.