  Given discrete subsets $\Lambda_j\subset {\Bbb R}^d$, $j=1,...,q$, consider
the set of windowed exponentials $\bigcup_{j=1}^{q}\{g_j(x)e^{2\pi i
<\lambda,x>}: \lambda\in\Lambda_j\}$ on $L^2(\Omega)$. We show that a necessary
and sufficient condition for the windows $g_j$ to form a frame of windowed
exponentials for $L^2(\Omega)$ with some $\Lambda_j$ is that $m\leq \max_{j\in
J}|g_j|\leq M$ almost everywhere on $\Omega$ for some subset $J$ of $\{1,...,
q\}$. If $\Omega$ is unbounded, we show that there is no frame of windowed
exponentials if the Lebesgue measure of $\Omega$ is infinite. If $\Omega$ is
unbounded but of finite measure, we give a sufficient condition for the
existence of Fourier frames on $L^2(\Omega)$. At the same time, we also
construct examples of unbounded sets with finite measure that have no tight
exponential frame.
