Our results support that decision aids are helpful for some cancer screening decisions. In these situations, DAs can increase knowledge, do not increase anxiety, and, in some circumstances, can influence the decision made. In contrast, there is very little data available evaluating decision aids for cancer-treatment-related decisions, and further evidence is still needed. The early stage of development of this field and the gaps in our knowledge determined by this systematic review underline the need for further research. A number of different areas were identified, such as developing a better understanding of how and when decisionmaking occurs; who is involved (clinician, patient, or others); and the extent of their involvement. The key features of quality decisionmaking need to be determined from patients and clinicians to help investigators develop appropriate interventions and to identify and prioritize outcome measures of effectiveness. Multicenter collaboration to formally set a research agenda is needed because integration of different research efforts in the field appears to be suboptimal. National or international collaboration would permit development of consensus about important basic concepts regarding decisionmaking, decision aids, and important outcomes.