Mesons beyond the naive quark model

We discuss theoretical predictions for the existence of exotic (non-quark-model) mesons and review prominent experimental candidates. These are especially the f(0)(1500) and f(0)(1710) mesons for the scalar glueball, f(J)(2220) for the tensor glueball, eta(1410) for the pseudoscalar glueball, f(0)(600), f(0)(980), a(0)(980), the still to be firmly established kappa(800) and the f(2)(1565) for q(2)(q) over bar (2) or two-meson states, and pi(1)(1400), pi(1)(1600) for hybrid states. We conclude that some of these states exist, offer our views and discuss crucial issues that need to be investigated both theoretically and experimentally. (C) 2003 Elsevier B.V. All rights reserved.