  In non-central high energy heavy ion collisions the colliding system posses a
huge orbital angular momentum in the direction opposite to the normal of the
reaction plane. Due to the spin-orbit coupling in strong interaction, such huge
orbital angular momentum leads to the polarization of quarks and anti-quarks in
the same direction. This effect, known as the global polarization effect, has
been recently observed by STAR Collaboration at RHIC that confirms the
theoretical prediction made more than ten years ago. The discovery has
attracted much attention on the study of spin effects in heavy ion collision.
It opens a new window to study properties of QGP and a new direction in high
energy heavy ion physics -- Spin Physics in Heavy Ion Collisions. In this
chapter, we review the original ideas and calculations that lead to the
predictions. We emphasize the role played by spin-orbit coupling in high energy
spin physics and discuss the new opportunities and challenges in this
connection.
