Quantifying the spin structure of the proton in terms of its quark and gluon constituents has been an animating goal of high-energy nuclear physics for decades. But a full understanding of the proton spin structure will necessarily require an extrapolation to small $x$, beyond measured data, where theoretical guidance is essential. In this work, we summarize recent developments in the theory and phenomenology of spin at small $x$, presenting results of a recent global analysis of inclusive and semi-inclusive deep inelastic scattering.