Title: The flat model structure on Ch(O) Author: James Gillespie Email: jrg21@psu.edu Abstract: Let Ch(O) be the category of chain complexes of O-modules on a topological space T (where O is a sheaf of rings on T ). We put a Quillen model structure on this category in which the cofibrant objects are built out of flat modules. More precisely, these are the dg-flat complexes. Dually, the fibrant objects will be called dg-cotorsion complexes. We show that this model structure is monoidal, solving the previous problem of not having any monoidal model structure on Ch(O). As a corollary, we have a general framework for doing homological algebra in the category O-MOD of O-modules. I.e., we have a natural way to define the functors Ext and Tor in O-MOD.