Time and again, spin has been a key element in the exploration of fundamental
physics. Spin-dependent observables have often revealed deficits in the assumed
theoretical framework and have led to novel developments and concepts. Spin is
exploited in many parity-violating experiments searching for physics beyond the
Standard Model or studying the nature of nucleon-nucleon forces. The RHIC spin
program plays a special role in this grand scheme: it uses spin to study how a
complex many-body system such as the proton arises from the dynamics of QCD.
Many exciting results from RHIC spin have emerged to date, most of them from
RHIC running after the 2007 Long Range Plan. In this document we present
highlights from the RHIC program to date and lay out the roadmap for the
significant advances that are possible with future RHIC running.