  Spin observables may reveal much deeper properties of non perturbative
hadronic physics than unpolarized quantities. We discuss here possible origins
of single spin asymmetries in DIS, absent in the elementary lepton-quark
interactions, and suggest strategies to isolate and understand some non
perturbative spin dependence of distribution and fragmentation functions. We
also discuss the polarization of hadrons produced in $e^+e^-$ annihilation at
LEP and show how final state $\qq$ interactions may give origin to non zero
values of the off-diagonal element $\rho_{1,-1}$ of the helicity density matrix
of vector mesons: some predictions are given for $K^{*0}, \phi$ and $D^*$ in
agreement with recent OPAL data. Possible analogous effects in DIS and other
processes are suggested.
