Accurate interpretation of posttherapeutic images obtained in radiation oncology patients requires familiarity with modern radiation therapy techniques and their expected effects on normal tissues. Three-dimensional conformal external-beam radiation therapy techniques (eg, intensity-modulated radiation therapy, stereotactic body radiation therapy), although they are designed to reduce the amount of normal tissue exposed to high-dose radiation, inevitably increase the amount of normal tissue that is exposed to low-dose radiation, with the potential for resultant changes that may evolve over time. Currently available internal radiation therapy techniques (eg, arterial radioembolization for hepatic malignancies, brachytherapy for prostate cancer and gynecologic cancers) also carry risks of possible injury to adjacent nontargeted tissues. The sensitivity of tissues to radiation exposure varies according to the tissue type but is generally proportional to the rate of cellular division, with rapidly regenerating tissues such as intestinal mucosa being the most radiosensitive. The characteristic response to radiation-induced injury likewise varies according to tissue type, with atrophy predominating in epithelial tissue whereas fibrosis predominates in stromal tissue. Moreover, changes in irradiated tissues evolve over time: In the liver, decreased attenuation at computed tomography and increased signal intensity at T2-weighted magnetic resonance imaging reflect hyperemia and edema in the early posttherapeutic period; later, veno-occlusive changes alter the hepatic enhancement pattern; and finally, fibrosis develops in some patients. In the small bowel, wall thickening and mucosal hyperenhancement predominate initially, whereas luminal narrowing is the most prominent feature of chronic enteropathy. Correlation of posttherapeutic images with images used for treatment planning may be helpful when interpreting complex cases.