Photonic crystals are periodically structured electromagnetic media, possessing photonic band gaps, (ie: ranges of frequency in which light cannot propagate through the structure). In this publication, the authors present current research in the study of the technology, theory and challenges of photonic crystals. Topics include photonic crystal tunable filters driven by an electrical control signal; recent patents on tunable photonic crystals; diamond photonics; design of high performance tunable reflectors by using 1D ternary photonic band gap structures; photonic crystal fibre non-linear characterisation and application to generate multi-wavelength brillouin fiber lasers; and a new multilayer structure for temperature stable performance in infrared optical filtering.