Advanced fibers developed in the 20th Century are now used in numerous applications from aerospace and automotive to sporting goods. However, there was no major breakthrough in advanced fiber properties since the last carbon fiber introduction by the Japanese more than three decades ago. Classical processing techniques for ultrahigh-performance polymer fibers, such as LC- and gel-spinning, rely on a combination of high polymer crystallinity and high degree of macromolecular alignment to achieve superior mechanical properties. As a consequence, advanced fibers such as Kevlar and Spectra possess extraordinary strength but low strain to failure and therefore low toughness. We have recently demonstrated that ultrafine continuous nanofibers can show extraordinary simultaneously high strength, modulus, AND toughness. Finest nanofilaments exhibited strength on the par with the best advanced fibers while exceeding their toughness by more than an order of magnitude. We showed that this unique and highly desirable mechanical behavior may be due to high degree of macromolecular alignment in conjunction with low crystallinity. In this lecture I will discuss how to exploit the discovered unique behavior and how to further improve nanofiber properties through processing. Several families of nanofibers were produced in broad diameter range by changing nanomanufacturing parameters. Reduction in crystallinity of nanofibers while maintaining or further increasing chain orientation was achieved through modified processing. This resulted in significant increases in strain to failure and toughness. Remarkably, NF strength and modulus also improved that was attributed to increased dope drawability. Notably, the major improvements in mechanical properties were observed in the intermediate diameter range that is much easier to produce and handle. Reported simultaneous improvements in mechanical properties of NFs can lead to inexpensive, simultaneously strong and tough composites and structures for safety critical applications. The proposed structural explanation of the discovered NF mechanical behavior challenges the prevailing paradigm in advanced fiber development calling for high polymer crystallinity and can lead to the entirely new class of advanced fibers with ultrahigh toughness, in addition to strength. Supporting additional results on several polymer and carbon systems, as well as applications of these novel fibers in hierarchical composites will be also discussed. 1. Dzenis Y. “Spinning Continuous Nanofibers for Nanotechnology”, Science, 304, 2004, 1917-1919 2. Dzenis, Y., "Structural Nanocomposites", Science, 2008, 319, 419-420 3. Ritchie, R.O.; and Dzenis, Y., "The Quest for Stronger, Tougher Materials", Science, 2008, 320, 448 4. Papkov, D., Zou, Y., Andalib, M.N., Goponenko, A., Cheng, S.Z.D., Dzenis, Y., "Simultaneously Strong and Tough Ultrafine Continuous Nanofibers,” ACS Nano, 2013, 7, 3324-3331 (cover of ACS Nano; highlighted in Nature, 495, 284, Materials Today, Science 360, Materials 360, NanoToday, NSF, and other outlets; cover of Nature special issue Review of Science) 5. Goponenko, A. and Dzenis, Y., “Role of Mechanical Factors in Applications of Stimuli-Responsive Polymer Gels – Status and Prospects” (feature article), Polymer, 2016, 101, 415–449 (cover Polymer, Vol 101, Sept 2016) 6. Maleckis, K. and Dzenis, Y. Continuous DNA Nanofibers with Extraordinary Mechanical Properties and High Molecular Orientation. 2018, 1800302, 1–8 7. Papkov, D., Pellerin, C., and Dzenis, Y. Polarized Raman Analysis of Polymer Chain Orientation in Ultrafine Individual Nanofibers with Variable Low Crystallinity. Macromolecules 2018, 51 (21), 8746–8751