The resolution attainable by an Inverse Synthetic Aperture Radar system is discussed. The image reconstruction method is based on recovering the target reflectivity function by an inverse Fourier transform of the received signal samples collected for different positions of radar and target. The resolution in range direction depends on the transmitted signal bandwidth, while the cross-range resolution is due to the variation of the aspect angle of the target and thus depends on the target motion. As an example, two relevant practical cases of target motion have been considered: the rectilinear motion of an aircraft and the rotatory motion of a ship (roll, pitch or yaw). The resolution that can be obtained and the plane on which the three-dimensional object is imaged are singled out from a theoretical point of view and by means a number of simulation tests.