"The lightest scalar mesons, with masses below 1 GeV have defied classification for nearly half a century. The isoscalar sigma plays a central role in nuclear physics and chiral dynamics, but its existence and its place in the quark model remain controversial. The narrow a_0 and f_0 states near K-Kbar threshold do not conform to standard q-qbar quark model expectations. A strange partner of the sigma, known as the $\kappa$ is even more elusive.

I will discuss the implications of the gluon exchange force between quarks for the lightest 0++ mesons. Its qualitative effect is to lower the mass of a nonet of q-q-qbar-qbar states. I will show how these states resemble the physical sigma, a_0, and f_0.

I will then present a recent study using lattice QCD which suggests that scalar q-q-qbar-qbar states occur generically near meson-meson thresholds. This provides strong evidence that a q-q-qbar-qbar is a major component in the physical light scalar mesons."