In these Webpages we introduce planar photonic crystals (PPCs). Two different approaches have been worked out ready for possible applications. i) Photonic slab structures consist of 2D photonic crystals of high-index material forming thin slabs. Under some conditions a total internal reflection happens at the interface between the slab and the lower-index materials lying above and below it. This is a way to obtain 3D light confinement in 2D PCs. ii) Photonic crystal-based gas-sensors utilize the decreased group velocity at photonic bands having very low slope. On the basis of theoretical computations this principle is worked out for 2D PCs of macroporous silicon. Because of the enhanced interaction of radiation and gas within the PC volume the gas-sensing system can be miniaturized.