The cover picture shows a new colorimetric thin film assembly for studying bacterial biofilms as described by R. Jelinek and co-workers in their Full Paper on page 752 ff. The white background corresponds to a transparent sol-gel layer constructed through a dip-coating technique. Within the sol-gel are domains of polydiacetylene—a unique chromatic polymer which undergoes dramatic color (and fluorescence) transformations. Initially, the polydiacetylene domains appear blue (and nonfluorescent), however, after specific accumulation of bacterial biofilms, the polydiacetylene regions turn intense red (and fluorescent). The polydiacetylene/sol-gel film assembly constitutes a powerful platform for monitoring biofilm formation in situ.