  Quarks play an active role in shaping the QCD vacuum structure. Being dual
carriers of both `color' and `electric' charges they also respond to externally
applied electromagnetic fields. Thus, in principle, the vacuum of strong
interactions influences higher order QED processes such as photon-photon
scattering. We survey here the current status of the understanding of the
vacuum structure of strong interactions, and take a fresh look at its
electromagnetic properties.
