  The importance of $S$-matrix unitarity in realistic meson spectroscopy is
reviewed, both its historical development and more recent applications. First
the effects of imposing $S$-matrix unitarity on meson resonances is
demonstrated in both the elastic and the inelastic case. Then, the static quark
model is revisited and its theoretical as well as phenomenological shortcomings
are highlighted. A detailed account is presented of the mesons in the tables of
the Particle Data Group that cannot be explained at all or only poorly in
models describing mesons as pure quark-antiquark bound states. Next the
earliest unitarised and coupled-channel models are revisited, followed by
several examples of puzzling meson resonances and their understanding in a
modern unitarised framework. Also, recent and fully unquenched lattice
descriptions of such mesons are summarised. Finally, attention is paid to
production processes, which require an unconventional yet related unitary
approach. Proposals for further improvement are discussed.
