The two main classes of simulations applicable to the modeling of high brightness electron beams are presented, namely the multi-particle integral method and the differential C.I.C. technique, and discussed with emphasis on the degree of self-consistency of the calculation and on the minimum equivalent temperature induced by all sources of errors into the phase space density distribution. These are among the main limiting issues which are of concern for any application aiming at low emittance and high quality beams. The impact of such limitations on the procedure of designing bright beam accelerators is also discussed, and the key role played by modelings based on semi-analytical descriptions is properly underlined, showing that these are quite useful tools, in particular for time dependent space charge dominated beams, in the general validation of much more complex (and time consuming) simulations.