In this lecture i want to survey some of the problems of the aerodynamics of re-entry, as they affect either purely earth-bound or space vehicles; and in particular, as they are affected by the trajectory the vehicles are designed to follow, or may find themselves upon due to some error of navigation. This is a self-set task and an ambitious one: but in truth it is an artful one, as it provides an excuse for me to leave the discussion of problems at a stage where their difficulties begin to defeat me.