I will discuss the modifications that occur in the bound state
dynamics of heavy quarks due to the presence of a medium. Effective
field theories both for non-relativistic bound states and for the
quark-gluon plasma will be the basic tools to guide this discussion.
For most of the talk, I will restrict myself  to weakly coupled states
and to a weakly coupled quark-gluon plasma, for which QED bound states
in an electron-positron plasma serve as a toy models. The role of the
relative velocity of the bound state with respect to the medium will
also be discussed. Finally, I will describe how this body of knowledge
can be embedded in a realistic setting to predict the yields of vector
states in heavy ion collision experiments.