In recent years, there is increasing interest in the research on optical fiber nanowires or microfibers (MFs) which are fibers with submicrometer- and nanometer- diameter. The size is tens to thousands of times thinner than the standard optical fiber. With the advantages of large evanescent fields, high nonlinearity, extreme flexibility and configurability and low-loss interconnection to other optical fibers and fiberized components，they have found potential applications in a wide range of fields of optical communications, sensing, lasers, biology and chemistry. This book discusses the fundamentals, applications and technological advances of nonlinear optics. (Imprint: Nova)