Keywords: Viscous boundary layers, Prandtl's boundary layer system, Navier-slip bounadry conditions, incompressible Navier-Stokes system. Abstract: In this talk, I will discuss some issues and recent results related to the viscous boundary layer theory. In particular, I will present a global existence result on classical soluition to the 2-dimensional unsteady Prandtl's boundary system; some convergence results on viscous solutions when the viscosity becomes small. Both non-slip and Navier-type slip boundary conditions will be studied.