IN SCIENTIFIC SCENARIOS, VOLUMETRIC DATA is frequently obtained by 3Dmeasurement, such as computerized tomography (CT) or magnetic resonance imaging (MRI). The volumetric objects used for visual arts and animation usually do not exist in the real world. They cannot be simply obtained by scanning an existing object. This chapter deals with modeling techniques that support the artist in creating volumetric models from scratch. Voxelization techniques allow volumetric representations to be created from polygonal surface descriptions by spatial discretization. Procedural modeling techniques are used to describe internal structures. Similar to photographed texture images for polygonal models, real 3D image data acquired by tomographic measurement can be used to supplement volumetric models.