Scanning transmission electron microscopy combined with monochromated electron energy-loss spectroscopy (STEM-EELS) has matured into a comprehensive multi-dimensional characterization platform that simultaneously maps atomic-scale structure, elemental distribution, valence states, and phonon dispersions. This review systematically examines the physical foundations and data-analysis strategies of STEM-EELS, highlighting three illustrative application domains: (i) catalytic interfaces, (ii) battery materials and (iii) quantum materials. We further address the synergy between 4D-STEM and EELS, which correlates crystallographic distortions with electronic fine-structure variations. We conclude by outlining future pathways: sub-meV energy resolution for magnon detection, liquid-helium stages for quantum-ground-state visualization, and real-time machine-learning-assisted spectral decomposition.

Scanning transmission electron microscopy; Electron energy loss spectroscopy; Catalysis; Energy materials; Quantum materials