It has long been known that starlight cannot account for the emission-line spectra of optical filaments in clusters. The filaments are surrounded by a plasma with both hot thermal and relativistic particles. The unusual emission-line spectra of the filaments, with anomalously strong atomic and molecular lines, are a natural consequence of the penetration of energetic particles into molecular gas. I describe a framework to predict the emitted spectrum. The chemical composition is within a factor of two of solar and dust is present. Both have implications for the origin of the filaments.