Freeform optical surfaces are of increasing interest in the design of modern imaging and illumination systems. However for the optical designer there are still methods missing in order to illustrate and quantify the effect of freeform surfaces in terms of aberrations or their impact on illumination performance. Phase space optics might offer an alternative way to understand freeform optics and their impact within optical systems. To prove this, phase space in geometrical optics is generally introduced, and some example systems are presented which allow us to relate phase space transformations to geometrical aberrations. In this way, we show that aberrations can be understood as nonlinear deformations of phase space, which need to be minimized in imaging designs or to be employed for homogenizing radiation in illumination designs.