Abstract: The contrast properties of computed tomography (CT) depend significantly on the X-ray energy spectrum used to measure the object. Conventional CT uses only a single energy spectrum and at times suffers from ambiguity so that two different materials can appear identical. Dual energy CT (DECT) uses two different energy spectra that can be used to remove this ambiguity. Although the basic concept of DECT is not new, its popularity has recently skyrocketed because of the availability of commercial implementations. We review the basic principles of DECT physics and estimation, as well as the technologies that have enabled DECT to enter modern clinical imaging. Applications of DECT abound, and we familiarize the reader with a selection of the most common clinical applications. Finally, we conclude by touching on two areas of current technical development: photon-counting detectors and synthetic CT.