The present invention is directed to methods of making nanofiber yarns. In some embodiments, the nanotube yarns comprise carbon nanotubes. Particularly, such carbon nanotube yarns of the present invention provide unique properties and property combinations such as extreme toughness, resistance to failure at knots, high electrical and thermal conductivities, high absorption of energy that occurs reversibly, up to 13% strain-to-failure compared with the few percent strain-to-failure of other fibers with similar toughness, very high resistance to creep, retention of strength even when heated in air at 450° C. for one hour, and very high radiation and UV resistance, even when irradiated in air.