What makes the proton spin?.

For the past 30 years, there has been an intense world-wide effort to understand how the quarks and gluons that make up the proton organize themselves to produce its spin of 1/2 hbar. ~The primary tool in this quest has been deep-inelastic scattering of polarized electrons and muons off polarized protons. ~A surprising discovery has been that the spins of the quarks and anti-quarks only contribute \textasciitilde 1/3 of the proton spin. ~During the past decade, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab has enabled a new, complementary probe, high-energy polarized pp collisions. ~The RHIC spin program has provided essential new insights, including evidence that the gluons in the proton are polarized and may even contribute a larger fraction of the proton spin than the quarks do. ~In this talk, I'll discuss what we've learned from the RHIC spin program, and where we are heading over the next several years.