  The vast majority of mesons can be understood as quark-antiquark states. Yet,
various other possibilities exists: glueballs (bound-state of gluons), hybrids
(quark-antiquark plus gluon), and four-quark states (either as
diquark-antidiquark or molecular objects) are expected. In particular, the
existence of glueballs represents one of the first predictions of QCD, which
relies on the nonabelian feature of its structure; this is why the search for
glueballs and their firm discovery would be so important, both for theoretical
and experimental developments. At the same time, many new resonances ($X,Y,$
and $Z$ states) were discovered experimentally, some of which can be well
understood as four-quark objects. In this lecture, we review some basic aspects
of QCD and show in a pedagogical way how to construct an effective hadronic
model of QCD. We then present the results for the decays of the scalar and the
pseudoscalar glueballs within this approach and discuss the future applications
to other glueball states. In conclusion, we briefly discuss the status of
four-quark states, both in the low-energy domain (light scalar mesons) as well
as in the high-energy domain (in the charmonia region).
