Borcherds Products on O(2 l) and Chern Classes of Heegner Divisors Around 1994 R. Borcherds discovered a new type of meromorphic modular form on the orthogonal group $O(2 n)$. These Borcherds products have infinite product expansions analogous to the Dedekind eta function. They arise as multiplicative liftings of elliptic modular forms on $(SL)_2(R)$. The fact that the zeros and poles of Borcherds products are explicitly given in terms of Heegner divisors makes them interesting for geometric and arithmetic applications. In the present text the Borcherds construction is extended to Maass wave forms and is used to study the Chern classes of Heegner divisors. A converse theorem for the lifting is proved. Introduction Vector valued modular forms for the metaplectic group. The Weil representation. Poincaré series and Einstein series. Non holomorphic Poincaré series of negative weight The regularized theta lift. Siegel theta functions. The theta integral. Unfolding against F. Unfolding against theta The Fourier theta lift. Lorentzian lattices. Lattices of signature (2 l). Modular forms on orthogonal groups. Borcherds products Some Riemann geometry on O(2 l). The invariant Laplacian. Reduction theory and L^p estimates. Modular forms with zeros and poles on Heegner divisors Chern classes of Heegner divisors. A lifting into cohomology. Modular forms with zeros and poles on Heegner divisors II.