  We adopt a $T$-matrix approach to study quarkonium properties and heavy-quark
transport in a Quark-Gluon Plasma. The $T$-matrix approach is well suited to
implement potential scattering and thus provides a common framework for
low-momentum transfer interactions in heavy-heavy and heavy-light quark
systems. We assume that the underlying potentials can be estimated from the
heavy-quark free energy computed in lattice QCD. We discuss constraints from
vacuum spectroscopy, uncertainties arising from different choices of the
potential, and the role of elastic and inelastic widths which are naturally
accounted for in the $T$-matrix formalism.
