  The proton is a composite object with spin one-half, understood to contain
highly relativistic spin one-half quarks exchanging spin-one gluons, each
possibly with significant orbital angular momenta. While their fundamental
interactions are well described by Quantum ChromoDynamics (QCD), our standard
theory of the strong interaction, nonperturbative calculations of the internal
structure of the proton based directly on QCD are beginning to provide reliable
results. Most of our present knowledge of the structure of the proton is based
on experimental measurements interpreted within the rich framework of QCD. An
area presently attracting intense interest, both experimental and theoretical,
is the relationship between the spin of the proton and the spins and orbital
angular momenta of its constituents. While remarkable progress has been made,
especially in the last decade, the discovery and investigation of new concepts
have revealed that much more remains to be learned. This progress is reviewed
and an outlook for the future is offered.
