  We investigate one- and multi-dimensional stability of noncharacteristic
boundary layers in the limit approaching a standing planar shock wave $\bar
U(x_1)$, $x_1>0$, obtaining necessary conditions of (i) weak stability of the
limiting shock, (ii) weak stability of the constant layer $u\equiv
U_-:=\lim_{z\to -\infty} \bar U(z)$, and (iii) nonnegativity of a modified
Lopatinski determinant similar to that of the inviscid shock case. For Lax
1-shocks, we obtain equally simple sufficient conditions; for $p$-shocks,
$p>1$, the situation appears to be more complicated. Using these results, we
determine stability of certain isentropic and full gas dynamical
boundary-layers, generalizing earlier work of Serre--Zumbrun and
Costanzino--Humphreys--Nguyen--Zumbrun.
