Within only a few years after its introduction in 1972, x-ray computed tomography (CT) established its important role in radiological diagnosis. Today, more than 20,000 clinical CT installations are in operation. CT technology is mature and its clinical use can be considered routine, but new scanning procedures and clinical applications are arising furtheron. Scanning of complete volumes in minimal time with spiral (helical) CT is one of the most important examples. Since the early nineties, conventional slice-by-slice scanning is more and more replaced by this new volume scanning technique. We review the principles of scanning and image reconstruction for axial CT and for spiral CT and present their performance characteristics and major new applications. CT started as a two-dimensional slice imaging modality; it now rapidly develops into a three-dimensional volume imaging technique.