Spin in High Energy Hadron Physics

The rapid development of high energy polarized lepton and hadron beams and polarized targets in the middle of the 20th century paved the way for a new generation of experiments designed to explore spin effects in the emerging field of Quantum Chromodynamics. These experiments unveiled a dynamic landscape of quark and gluon interactions that has radically changed our simple picture of the proton and revealed novel correlations between the spin of the hadron and the parton flavor, spin and momentum, both in the initial bound state as well as in the hadronization process. This talk will present a historical overview of the evolution of spin structure in high energy hadron physics, highlighting the major discoveries as well as the new and persistent mysteries. Opportunities for future measurements at existing and future facilities will be discussed.