Introduction: voxel density value in CBCT is used regularly in many clinical aspects such as virtual implant planning. Many researchers studied factors affecting it and their significance. Most of the results obtained are contradicting and /or inconclusive. Objectives: to compile papers inquiring effect factors affecting voxel grey values and their significance. Methodology: internet search was done on two databases using MeSH terms. Resulting studies went through two level screening according to predetermined inclusion and exclusion criteria. Results: A total of 27 studies were included. Total number of factors, which were tested in the included papers, were eighteen. Conclusion:Significantly affecting factors are (FOV, mA, Objects position inside the FOV, Objects exo-mass, kVp, time between exposure, number of basis, adjacent air to the ROI).Insignificantly affecting factors are (exposure parameters, software, exposure dose, presence of teeth, presence of metallic post in or out the FOV). Inconclusive factors are (Machines model, voxel size, objects mass, receptor type, exposure time). Grey levels in CBCT which is known by Hounsfield Unit of CBCT is not reliable nor repeatable.