Theoretical Overview of Longitudinal Spin Physics

Opening the afternoon session of mainly longitudinal spin physics results, my overview will cover the theoretical underpinnings dating from the {\it Spin Crisis} to recent spin-related concepts, models, and expectations. The global analysis of experimental results -- such as to be presented after my talk -- provides a largely model-independent and coherent framework to extract the relevant theoretical information and the uncertainties thereof. I will outline the knowns and unknowns (incl.\ debatables) of the underlying building blocks: Perturbative expansion in terms of twist and coupling (higher orders) as well as statistical and computational tools.