The technology for using the telephone to deliver interventions to improve health behavior and health care has expanded dramatically and exciting innovations continue to emerge. These developments enable the telephone to be used to deliver individualized services to a broad cross section of target groups, while minimizing logistic and system barriers, and cost. However, as telephone-delivered interventions have proliferated, a number of questions remain concerning the elements that are essential to their effectiveness. We provide a broad overview of telephone interventions, published in the past 10 years to identify gaps in knowledge about these services, and make recommendations for future program development and evaluation.