The bright blue colour of the Cyanophrys remus butterfly has its origin in the unique periodic nanostructures in its wings, which are called ‘photonic crystals’. For more than two decades, engineers in Photonics technology are trying to understand the operation of such photonic crystals. Their unique properties enable new ways to control, guide and manipulate light leading to new applications including optical chips for future light-powered computers and for medical analysis, new solar cells and new light sources.