motivation for Peccei-Quinn symmetry and axions, with their profound physical and cosmological implications. The ratio $m_u/m_d$ partially determines the axion coupling, and will play a vital role in pinning down the underlying microscopic parameters if and when axions are observed. The proton-neutron mass difference, whose value makes all the difference for nuclear stability and for stellar and cosmic nucleosynthesis – and thereby to the structure and composition of matter – of course depends quite directly upon $m_u -m_d$. Finally, there is the prospect of making precise quantitative predictions for appropriate measurable isospin-violating processes. Especially promising in this regard are transitions between heavy quark-heavy antiquark states by soft $\pi^0$ emission. Conversely, these processes allow a different, and potentially cleaner, path to the determination of $m_u -m_d/m_u +m_d$ than the traditional ways involving electromagnetic mass differences or light hadron processes.

[$m_s$]{} The strange quark mass plays a singular role in QCD. For all the other quark masses, it makes good sense to expand either in $m/\Lambda$ or $\Lambda/m$, where $\Lambda$ is the primary QCD scale, vaguely in the neighborhood of 300 MeV. For the strange quark neither expansion is clearly appropriate. A fundamental question that I find quite interesting, and which is 