Modern radiotherapy now involves the use of more types of image data in a more quantitative fashion than any other field of medicine. Image data are acquired and analyzed at each step of the radiotherapy process; initial diagnosis, treatment planning, treatment delivery and monitoring the patient during and after therapy. The overall goal is to develop as complete and accurate a model of the patient as possible. To carry this out in quantitative fashion software tools are required to register time-series and multimodality image data and to integrate the information provided by these data as well as estimates of delivered doses. In addition to understanding how to use the tools and appreciate and access their limitations, clinical strategies for acting on the results are essential. This session will review the basic techniques and practical use and limitation of software for simple and complex image registration and data fusion and discuss their clinical use for planning, delivery and plan adaptation. At the conclusion of this activity, the learner will be able to do the following: 1. Describe the techniques and practical use of image registration (of time series and multimodality data). 2. Explain the techniques and practical use of data fusion for contour propagation and dose accumulation. 3. Identify how the techniques of image registration and data fusion are used for clinical decisions in planning, delivery and adaptation of radiotherapy.