We are concerned about problems in plasmas and fluids with outgoing flow velocity at the physical boundary. We consider the Vlasov-Poisson equation on a half line with outflow (completely absorbing) boundary conditions, and present a result on the nonlinear stability of a family of stationary solutions. This is a model of the formation of a plasma boundary layer (sheath). If time permits, I will also present a result about an outflow problem on the compressible Navier-Stokes equation. This is joint work with M. Suzuki and M. Takayama.