The phenomenology of transport barrier formation is reviewed with a focus on physics that may be common to the edge and the core. To this end, the framework of velocity shear reduction of turbulence-induced fluxes, applied to the edge for some time, is studied in the light of measurements of core bifurcation dynamics and recent tests of causality. Also, the possible role of the magnetic shear structure in facilitating core barrier formation is examined. Experimental and theoretical challenges for developing a predictive capability for reactor-relevant conditions are highlighted by recent observations of spontaneous electric field shear generation far removed from edge effects, and efforts to characterize the plasma edge at and across the L-H transition.