The impact of multi-loop calculations on precise determinations of charm- and bottom-quark masses
and the strong coupling constant is discussed. Recent $N^3LO$ calculations, combined with new
precision data, lead to a significant reduction of the errors. The results of these analyses,
$m_c(3~\mbox{GeV}) = 0.986(13)~\mbox{GeV}$, $m_b(10~\mbox{GeV}) = 3.609(19)~\mbox{GeV}$
and $\alpha_s(M_Z)=0.1198(15)$
constitute the most precise and presently only $N^3LO$ determinations of these fundamental quantities.
