  We discuss some experimental and theoretical results on single spin
asymmetries (SSA) in high energy lepton-hadron and hadron-hadron reactions. In
particular, recent results on meson SSA obtained by HERMES are considered in
detail. We also discuss the SSA results obtained recently by COMPASS, as well
as those from BRAHMS, PHENIX and STAR. Special attention is paid to a possible
nonperturbative QCD mechanism that might be responsible for the observed meson
SSA. This mechanism originates from the spin-flip quark-gluon chromomagnetic
interaction induced by the complex topological structure of the QCD vacuum. We
argue that in semi-inclusive deep-inelastic scattering a large SSA is expected
not only for mesons but also for baryons due to strong nonperturbative final
state interactions between $ud$-diquark and $u$-quark in the fragmenting
proton.
