Photonic crystal fibers as photocatalytic microreactors! The cover picture shows a photonic crystal fiber (PCF), which can guide light in a hollow core filled with reactants and catalysts. In their Concept paper on p. 641 ff., researchers from the Excellence Cluster Engineering of Advanced Materials in Erlangen describe recent progress on using PCFs as reactors with nL volumes, and discuss the further opportunities raised by the unique technology. The excellent overlap between guided light and fluids in the core allows for in situ spectroscopy and photochemical activation. Compared to spectroscopy using conventional cuvettes of centimeter dimensions, interaction lengths up to several meters can easily be achieved, making it possible to detect compounds in ultra-low concentrations. In addition, the small core diameter means that ultra-high power densities can be reached at moderate laser powers, which is advantageous for photochemical activation.