Technological advances in medical imaging have tremendously driven the continuous improvement in precision and accuracy of tumor detection and treatment planning in radiation oncology. The use of x-ray computed tomography (CT) scans for treatment preparation is common practice in radiotherapy. Dual-energy CT (DECT), the acquisition of two CT scans with different x-ray spectra, provides the opportunity to quantitatively determine material parameters by exploiting the additional tissue information derived from DECT. Since the clinical introduction of DECT for diagnostic imaging 15 years ago, a multitude of research studies identified various promising benefits of DECT in radiation oncology with a favorable effect on the entire radiotherapeutic chain from delineation, treatment planning to dose calculation. This presentation will give an overview on available clinical applications and the current state of research in radiation oncology as well as highlight promising future developments.