The explosion of new imaging technologies such as X ray computed tomography (CT), ultrasound (US), positron emission tomography (PET), and nuclear magnetic resonance imaging (NMR) has forced a major change in radiation therapy treatment planning philosophy and procedures. Modern computer technology has been wedded to these new imaging modalities, making possible sophisticated radiation therapy treatment planning using both the detailed anatomical and density information that is made available by CT and the other imaging modalities. This has forced a revolution in the way treatments are planned, with the result that actual beam configurations are typically both more complex and more carefully tailored to the desired target volume. This increase in precision and accuracy will presumably improve the results of radiation therapy.