Electrochromic windows are reviewed with a focus on the chemistry, electrochemistry, stability and manufacturing of the different window varieties. Windows are divided into three classes; battery-like, with open circuit memory and solution phase and hybrid designs, which are self erasing and require a maintenance current during caloration. Construction of windows includes all thin film solid state structures and devices containing polymeric or gel electrolytes. The stability and manufacturing considerations include the presence of water, organic materials and the nature of the main transportable ion (protons or Li+). Examples from this laboratory representing protic and aprotic systems are presented, with structures HxWO3\acidic polyelectrolyte\Ir oxide and LixWO3[Li2O-B2O3\LiyCrO2+x respectively.