  We define and study a Weil-\'etale topos for any regular, proper scheme $X$
over $\Spec(Z)$ which has some of the properties suggested by Lichtenbaum for
such a topos. In particular, the cohomology with $R$-coefficients has the
expected relation to $\zeta(X,s)$ at $s=0$ if the Hasse-Weil L-functions
$L(h^i(X_Q),s)$ have the expected meromorphic continuation and functional
equation. If $\X$ has characteristic $p$ the cohomology with $Z$-coefficients
also has the expected relation to $\zeta(X,s)$ and our cohomology groups
recover those previously studied by Lichtenbaum and Geisser.
