Title: Algebraic Geometry over model categories (a general approach to derived algebraic geometry) Authors: Bertrand Toen and Gabriele Vezzosi AMS Classification numbers: 14A20; 18G55; 55P43; 55U40;18F10 Submitted to the xxxLANL as math.AG/0110109, October 10, 2001 Addresses of authors: Bertrand Toen, Laboratoire J. A. Dieudonne, UMR CNRS 6621, Universite' de Nice-Sophia Antipolis, France. toen@math.unice.fr Gabriele Vezzosi, Diprtimento di Matematica, Universita' di Bologna, Italy, vezzosi@dm.unibo.it Included gzipped .ps file ABSTRACT: For a (semi-)model category M, we define a notion of a ''homotopy'' Grothendieck topology on M, as well as its associated model category of stacks. We use this to define a notion of geometric stack over a symmetric monoidal base model category; geometric stacks are the fundamental objects to "do algebraic geometry over model categories". We give two examples of applications of this formalism. The first one is the interpretation of DG-schemes as geometric stacks over the model category of complexes and the second one is a definition of etale K-theory of E_{\infty}-ring spectra. This first version is very preliminary and might be considered as a detailed research announcement. Some proofs, more details and more examples will be added in a forthcoming version.