We will review some recent advances in the study of the Prandtl system and the inviscid limit. In particular, reversal occurs after the Goldstein singularity (the separation point), and is characterized by regions in which the velocity changes sign. The classical point of view of treating the stationary Prandtl system as an evolution equation in the tangential variable x completely breaks down. Instead, we view the problem as a quasilinear, mixed-type, free-boundary problem. Joint works with Nader Masmoudi.