  Heavy quarkonium suppression was proposed long ago as a signal of the
formation of a deconfined phase in heavy ion collisions. Originally the
mechanism responsible for this suppression was thought to be color screening.
However perturbative computations and recent lattice studies suggest the
existence of an imaginary part of the potential which could have a more
important role for suppression than screening. In this work we review some
general aspects of effective field theories for heavy quarkonium in the medium
and discuss the physical phenomena behind the imaginary part of the potential
and the decay width of heavy quarkonium and their corresponding cross sections.
