Switchable mirrors modulate incident light by changes in the reflectivity of thin films that are converted by chemical reactions from a metallic reflective state to a semiconducting transparent state. The reactions may be effected by exposure to a gas, by treatment with a liquid reagent, or by electrochemical means. The changes in optical properties coincide with phase transformations. In the examples reviewed here, the reactions are accompanied by substantial volume changes and atomic rearrangement. Bismuth, antimony, and antimony alloy films are switched by electrochemical lithiation. Copper films are electrochemically oxidized to Cu(I) and Cu(II) oxides, which are transparent and opaque, respectively. Morphological changes in all films during cycling lead to degradation in performance without loss of active material. Strategies to improve the cycling stability of these systems are discussed.