An ever-increasing extent of the wake behind a moving wing implies an ever-increasing energy of the flow field. The rate of the energy growth can be directly translated into the work done by the wing in the course of its motion, and when the motion is steady, it can be translated into the pressure (induced) drag acting on the wing. The rate of the energy growth can be conveniently estimated by examining the flow field in the transverse plane far downstream of the wing, aka the ‘Trefftz plane’. Energy considerations in the analysis of aerodynamic losses is the subject matter of this chapter.