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Two fast, CCD-based three-dimensional CT scanners for in vivo applications have been developed. One is designed for small laboratory animals and has a voxel size of 20 pm, while the other, having a voxel size of 80 pm, is used for human examinations. Both instruments make use of a novel multiple fan-beam technique: radiation from a line-focus X-ray tube is divided into a stack of fan-beams by a 28 pm pitch foil collimator. The resulting wedge-shaped X-ray field is the key to the instrument's high scanning speed and allows to position the sample close to the X-ray source, which makes it possible to build compact CT systems. In contrast to cone-beam scanners, the multiple fan-beam scanner relies on standard fan-beam algorithms, thereby eliminating inaccuracies in the reconstruction process. The projections from one single rotation are acquired within 2 mm and are subsequently reconstructed into a 1024 x 1024 x 255 voxel array. Hence a single rotation about the sample delivers a 3D image containing a quarter of a billion voxels. Such volumetric images are 6.6 mm in height and can be stacked on top of each other. An area CCD sensor bonded to a fiber-optic light guide acts as a detector. Since no image intensifier, conventional optics or tapers are used throughout the system, the image is virtually distortion free. The scanners' high scanning speed and high resolution at moderately low radiation dose are the basis for reliable time serial measurements and analyses.