X-rays have not yet been used to their fullest potential in a range of application areas, especially the agricultural and food industries. As a sub-surface technique, X-ray radiation can give three-dimensional information about the spatial density differences and changes of the atomic number within a sample. To get a better understanding of the potential of X-ray imaging most of the physical processes involved can be described by certain computational simulation programs, for which a division can be made between linear attenuation and full photon history descriptions. Specific areas of interest are small changes in the density of materials which consist of low atomic number elements, and the use of scattered photon information. Specific results from both the linear attenuation and the full photon history approaches are presented, their usefulness for X-ray imaging, and application areas are discussed. The simulation approaches concentrate on the detection of perspex, a soft plastic, and cellulose in a water environment. Possible applications of these techniques within the agricultural and food industries, such as foreign-body detection, quality control and food processing, are indicated.