Cost-effectiveness analysis (CEA) is an established analytic technique that can inform health care systems on how to prioritize different treatment approaches. In this session, the background and theory of cost-effectiveness analysis will be described to provide the audience with an understanding of the basic issues involved in the field and how it relates to radiation oncology. The effectiveness measure, the quality-adjusted life year (QALY), will be defined, with a theoretical explanation of why it is used in CEA. The costs of radiation therapy will be discussed to provide insight into the drivers of escalating treatment costs. Finally, the two core methods for performing cost-effectiveness analyses — decision models and trial-based studies — will be described, with case studies provided to showcase "real-world" examples of how these analyses are performed. At the conclusion of this activity, the learner will be able to do the following: 1. Describe the importance of cost-effectiveness studies in radiation oncology. 2. Recognize the different costs of radiation therapy. 3. Explain the two basic methods for performing cost-effectiveness analyses: model-based and trials-based analysis.