We review and contrast key technologies developed to address the optical components market for telecom applications. We first look at different material systems, compare their properties, and describe the functions achieved to date in each of them. The material systems reviewed include silica fiber, silica on silicon, silicon on insulator, silicon oxynitride, sol-gels, polymers, thin film dielectrics, lithium niobate, indium phosphide, gallium arsenide, magneto-optic materials, and birefringent crystals. We then look at the most commonly used classes of technology and present their pros & cons as well as the functions achieved to date in each. The technologies reviewed include passive, actuation, and active technologies. The passive technologies described include fused fibers, dispersion-compensating fiber, beam steering (e.g., AWG), Bragg gratings, diffraction gratings, holographic elements, thin film filters, photonic crystals, microrings, and birefringent elements. The actuation technologies include thermo-optics, electro-optics, acousto-optics, magneto-optics, liquid crystals, total internal reflection technologies (e.g., bubble technology), and mechanical actuation (e.g., moving fibers, MEMS). We finally describe active technologies including heterostructures, quantum wells, rare earth doping, Raman amplification, and semiconductor amplification. We also investigate the use of different material systems and technologies to achieve building block functions including lasers, amplifiers, detectors, modulators, polarization controllers, couplers, filters, switches, attenuators, non-reciprocal elements for isolators and circulators, wavelength converters, chromatic dispersion compensators, and polarization mode dispersion (PMD) compensators. Some of the technologies presented are established in the industry, others have recently been proven to be commercially viable, and some others are still under development in laboratories.