Molecular carbon chains have attracted much interest for more than 130 years, but the length of chains is limited to 44 atoms even by sophisticated chemical synthesis. Recently, the artificial synthesis of long linear carbon chains, "carbynes," has revived, and their existence was firmly substantiated using the latest advanced analytical methods, such as high-resolution electron microscopy and Raman scattering spectroscopy. Until the 1980s, graphite and diamond were the well-known allotropic forms of elemental carbon, which were two-dimensional (2D) and 3D crystals, respectively. Carbyne is the ultimate 1D nanowire with atomic diameter and its synthesis has opened prospects for versatile properties of carbon materials. Carbyne is a 1D semiconductor with a direct transition energy gap and interesting properties such as extreme mechanical strength are expected from it. This book comprehensively reviews and describes the latest chemical and physical synthesis methods, theoretically predicted properties, and possible applications of carbyne.