  This document summarizes recent achievements of the RHIC spin program and
their impact on our understanding of the nucleon's spin structure, i.e. the
individual parton (quark and gluon) contributions to the helicity structure of
the nucleon and to understand the origin of the transverse spin phenomena. Open
questions are identified and a suite of future measurements with polarized
beams at RHIC to address them is laid out. Machine and detector requirements
and upgrades are briefly discussed.
