  Let $\Omega$ be an unbounded, pseudoconvex domain in $\Bbb C^n$ and let
$\varphi$ be a $\mathcal C^2$-weight function plurisubharmonic on $\Omega$. We
show both necessary and sufficient conditions for existence and compactness of
a weighted $\bar\partial$-Neumann operator $N_\varphi$ on the space
$L^2_{(0,1)}(\Omega,e^{-\varphi})$ in terms of the eigenvalues of the complex
Hessian $(\partial ^2\varphi/\partial z_j\partial\bar z_k)_{j,k}$ of the
weight. We also give some applications to the unweighted $\bar\partial$-Neumann
problem on unbounded domains.
