Photonic crystals whose properties can be manipulated with an external magnetic field have been made by self-assembly of magnetic nanoparticles around larger polystyrene spheres into opaline, long-range-ordered lattices (see Figure for details of a sample after calcination). Optical properties of such crystals (e.g., wavelength of diffracted light) can be tuned by magnetically rotating the crystals relative to the incident light (see also images on the front cover).