Our purpose is to study the cohomological properties of the Rees algebras of a class of ideals generated by quadrics. For all such ideals I ⊂ R = K [x, y, z] we give the precise value of depth R[It] and decide whether the corresponding rational maps are birational. In the case of dimension d ≥ 3, when K = R, we give structure theorems for all ideals of codimension d minimally generated by d+1 2 −1 quadrics. For arbitrary fields K, we prove a polarized version.