  In this paper we show how the stability of Prandtl boundary layers is linked
to the stability of shear flows in the incompressible Navier Stokes equations.
We then recall classical physical instability results, and give a short
educational presentation of the construction of unstable modes for Orr
Sommerfeld equations. We end the paper with a conjecture concerning the
validity of Prandtl boundary layer asymptotic expansions.
