Medical decision making is sometimes viewed as a relatively simple process in which a decision may be made by the patient, by the physician, or by both patient and physician working together. This two-dimensional portrayal eclipses the important role that others, such as other professionals, family, and friends, may play in the process; as an example of this phenomenon, we trace the evolution of a decision of a teenager with cancer who is contemplating discontinuing chemotherapy. This example also shows how a decision can usefully be understood as consisting of a number of identifiable substeps--what we call the "microstructure" of the decision. These steps show how the physician can play an important role without usurping the patient's rightful decisional authority.