Algorithms for 3D tomography are still under development. The main problem is that the data entering explicit inversion formulas are usually not available. Thus the main task in 3D tomography is the derivation of inversion formulas which use only the data measured by a specific imaging device. It is clear that these formulas are taylored to the imaging device. In this section we restrict ourselves to the derivation of exact or approximate inversion formulas. The implementation in a discrete setting can be done along the lines of the 2D algorithms.