The impact that 3D (x,y,z) and 4D (x,y,z,time) X-ray experimental methods have had on the understanding of recrystallization of deformed metals are presented. Well-planned experiments on nucleation and grain boundary migration are reviewed with focus on information gained, which could not have been obtained by traditional 2D methods. This includes new results on particle stimulated nucleation, and effects of the deformation microstructure morphology on boundary migration. It has however also been found that the 3D/4D X-ray measurements sometimes happen to reveal phenomena, which were not expected and therefore not planned to investigate. This includes new results on dislocation structures and residual stress inside recrystallizing grains. Finally, results of 1-1 combinations of experimental results and phase field / molecular dynamics simulations of recrystallization are discussed, and a refined nucleation theory is suggested.