  We recall the definitions and the basic properties of the transversity
distributions $h^{q,\bar q}_1(x,Q^2)$ and the corresponding nucleon tensor
charges $\delta q(Q^2)$. We briefly comment on different estimates from several
phenomenological models and on the future possible measurements with the
polarized $pp$ collider at RHIC-BNL. Recent works on the $Q^2$-evolution of
$h^{q,\bar q}_1(x,Q^2)$ are also discussed and their implications on a very
useful positivity bound.
