The fact that light absorption and emission depend on the environment of the ions and molecules which absorb or emit light can be used to draw conclusions about the structure of liquids, glasses, and crystals. This method supplements, to a certain extent, information gained from other sources, such as x‐ray diffraction data. The method is illustrated by several examples, like the influence of thermal history on the structure of glass and the constitution of the yellow fluorescing zinc orthosilicate.