  An unambiguous definition of meson resonance masses requires a description of
the associated phase shifts in terms of a manifestly unitary $S$-matrix and its
complex poles. However, the commonly used Breit-Wigner (BW) parametrisations
can lead to appreciable deviations. We demonstrate this for a simple elastic
resonance, viz. $\rho(770)$, whose pole and BW masses turn out to differ by
almost 5 MeV. In the case of the very broad $f_0(500)$ and $K_0^\star(700)$
scalar mesons, the discrepancies are shown to become much larger, while also
putting question marks at the listed PDG BW masses and widths. Furthermore,
some results are reviewed of a manifestly unitary model for meson spectroscopy,
which highlight the potentially huge deviations from static model predictions.
Finally, a related unitary model for production amplitudes is shown to explain
several meson enhancements as non-resonant threshold effects, with profound
implications for spectroscopy.
