The field of radiation therapy has made dramatic technical advances over the past 20 years. 3D conformal radiotherapy, intensity-modulated radiation therapy and proton beam therapy have all been developed in an attempt to improve the therapeutic ratio: higher cure rates with lower toxicity. Unfortunately, although the costs of radiation therapy are certainly increasing, it is unclear whether its clinical benefit has also improved. Cost-effectiveness analyses are designed to formally evaluate the cost of a treatment relative to an associated change in quality-adjusted survival. As the cost of oncologic care is increasing, it is critically important to assess the cost-effectiveness of radiation therapy. This article will describe the issues surrounding the delivery and cost of radiation therapy, and it will summarize the work that has been done to evaluate the use of cost-effectiveness in radiation oncology.