Free divisors are a class of hypersurfaces in non singular ambient spaces which are characterized by differential properties. This mysterious class contains discriminants of versal deformations, some hyperplane arrangements, and some projective plane curves whose singularities have particular properties. The goal of this meeting is to introduce the audience to free divisors, to algebraic and topological properties of hyperplane arrangements and in particular to Terao’s conjecture about the freeness of hyperplane arrangements. Hyperplane arrangements play a translation role between some geometric problems or D-modules problems and some combinatorial problems. This role is somehow similar to the role played by toric varieties in translating other type of geometric problems to other aspects of combinatorics.