Electrochromic (EC) materials that change their optical transmittance under an external electrical field may form the basis of “smart windows”, which are of great interest in forthcoming building technologies. Nanostructured EC materials or assemblies have revealed remarkable improvement on colouration efficiency and switching time due to their small featured sizes and large surface areas. Here, the recent progress of nanoelectrochromics is reviewed; the scientific and technical issues related to material preparation and device assembly for large-area and large-scale window applications are discussed.