  We discuss the phenomenology of scalar and pseudoscalar mesons, emphasizing
those which do not carry manifest flavor quantum numbers. Many of the
properties of these mesons are still not fully understood. Some of them
probably do not have the usual two-quark (quark-antiquark) structure, but may
be mixed with glueball states or other exotics. %, hybrids, or four-quark
states. We construct or discuss simple models for these mesons and point out
which measurements can shed light on their composition.
