Lichtenbaum conjectured the existence of a Weil-��tale cohomology in order to describe the vanishing order and the special value of the Zeta function of an arithmetic scheme $\mathcal{X}$ at $s=0$ in terms of Euler-Poincar�� characteristics. Assuming the (conjectured) finite generation of some ��tale motivic cohomology groups we construct such a cohomology theory for regular schemes proper over $\mathrm{Spec}(\mathbb{Z})$. In particular, we obtain (unconditionally) the right Weil-��tale cohomology for geometrically cellular schemes over number rings. We state a conjecture expressing the vanishing order and the special value up to sign of the Zeta function $��(\mathcal{X},s)$ at $s=0$ in terms of a perfect complex of abelian groups $R��_{W,c}(\mathcal{X},\mathbb{Z})$. Then we relate this conjecture to Soul��'s conjecture and to the Tamagawa number conjecture of Bloch-Kato, and deduce its validity in simple cases.

53 pages. To appear in Duke Math. J