  Recent developments in the treatment of a system of heavy quarks close to
their production threshold are discussed. Particular emphasis is put on the
next-to-next-to-next-to-leading order (N$^3$LO) corrections to the energy
levels and the wave function and their effect on the cross section
$\sigma(e^+e^-\to t\bar{t})$. Furthermore, the resummation of logarithmic
corrections within the framework of potential non-relativistic QCD (pNRQCD) are
considered.
