  It has been realized for quite a long time that single-spin experiments, in
which one of the colliding objects is transversely polarized, can be helpful in
studying the properties of strong interaction in general and in testing Quantum
Chromodynamics (QCD) in particular.
  Striking effects have been observed in the past few years which deviate
drastically from the expectation of the perturbative QCD parton model.
  These effects have received much attention.
  New experiments of the similar type are underway and/or planned.
  Different theoretical attempts have been made to understand these effects.
  In this review, the special role played by singly polarized high-energy
hadron-hadron collisions in High Energy Spin Physics is emphasized.
  Characteristics of the available data for inclusive hadron productions are
briefly summarized.
  Different theoretical approaches for such processes are reviewed with special
attention to a non-perturbative model which explicitly takes the orbital motion
of the valence quarks and hadronic surface effects into account.
  The connection between such asymmetries and hyperon polarization in
unpolarized reactions is discussed.
  An example of the possible application of such experimental results in other
processes is given.
