Hydrogel Smart Windows In article number 2211716, Xiancai Jiang, Jun Fu, and co-workers report a novel thermochromic hydrogel-based smart window for all-weather building-temperature regulation in diverse climates. The hydrogel smart windows spontaneously transform from transparent to opaque upon temperature changes. The outdoor solar radiation in summer and indoor thermal radiation in winter are both regulated by the smart window to modulate the building temperature and save energy.