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Metamaterials are artificially engineered structures with unusual electromagnetic properties. In this article, we review the implementation of isotropic metamaterials that exhibit a negative permittivity and a negative permeability, thus leading to a negative index of refraction. Specifically, the article focuses on transmission-line metamaterials, which are planar structures comprising a network of distributed transmission lines loaded periodically with inductors, L, and capacitors, C, in a "high-pass" configuration. The periodic unit cell is much smaller than the wavelength, thus leading to an effective medium in which the lumped loading elements can be either discrete (chip) or printed. Based on such negative-refractive-index transmission-line (NRI-TL) metamaterials, several RF/microwave devices are presented, including microwave lenses that can overcome the diffraction limit, compact phase-shifting lines and associated broadband series-fed power dividers, electrically small antennas, antenna feed networks and baluns, backward leaky-wave antennas, and high-directivity coupled-line couplers and reflectometers.