  We review how RHIC is expected to deepen our understanding of the spin
structure of longitudinally and transversely polarized nucleons. After briefly
outlining the current status of spin-dependent parton densities and pointing
out open questions, we focus on theoretical calculations and predictions
relevant for the RHIC spin program. Estimates of the expected statistical
accuracy for such measurements are presented, taking into account the
acceptance of the RHIC detectors.
