Carbon nanothreads, formed by high-pressure polymerization of solid benzene, form as each benzene makes six bonds to each neighbors, transforming from sp2 to sp3 bonding. These systems, intermediate between polymers and nanowires, provide a fascinating example of one-dimensional sp3 carbon. “Degree 6” threads, with complete saturation, are large-gap insulators with extreme mechanical properties. Degree 4 threads, with one residual double bond per benzene, have prospects for interesting charge transport behavior and optical properties. Recent experimental results will be interpreted to narrow down the possible atomic bonding patterns within these threads, and prospects for generalizing the synthesis to related aromatic precursors will be discussed.