Spontaneous emission characteristics are not an inherent property of an emitter, but may be modified by a nanostructured environment surrounding the emitter. The use of one-, two-, and three-dimensional photonic crystals allows for significant and practical control of the spontaneous emission properties in optoelectronic communications devices. Particularly, one-dimensional photonic crystal structures, which are also known as microcavities, already are used in commercial devices. Two- and three dimensional photonic crystals permit a more comprehensive control of the spontaneous emission properties. By employing photonic crystals the device efficiency is enhanced, the angular radiation pattern can be engineered, and faster devices are achieved by decreasing the radiative lifetime. Photonic-crystal defect structures allow further engineering of the emission properties. Important applications for spontaneous emission control are light-emitting diodes, lasers, and single-photon sources.