Nonlinear fiber optics focuses on nonlinear optical phenomena occurring within optical fibers. The field began in 1961 with the use of a ruby laser to generate second-harmonic radiation in crystals, but the use of optical fibers as a nonlinear medium became feasible after 1970 when fiber losses were reduced to below 20 dB/km. Early studies in 1972 explored stimulated Raman and Brillouin scatterings in single-mode fibers, followed by other nonlinear effects like self- and cross-phase modulation and four-wave mixing. By 1989, the field had advanced enough to warrant a dedicated book, and it has continued to grow significantly in the 1990s, driven by advancements in high-capacity fiber-optic communication systems. The chapter highlights the unique properties of optical fibers that make them useful for nonlinear optics, particularly single-mode fibers. These fibers consist of a central core surrounded by a slightly lower refractive index cladding, made of fused silica with dopants to enhance or reduce the refractive index. The guiding properties of optical fibers are characterized by the dimensionless parameter \( V \), which determines the number of modes supported. Single-mode fibers have \( V < 2.405 \) and are used extensively in optical communications. The chapter also discusses fiber nonlinearities, which arise from the third-order susceptibility \(\chi^{(3)}\). Key nonlinear effects include third-harmonic generation, four-wave mixing (FWM), and nonlinear refraction, with nonlinear refraction being the most significant due to its role in the intensity-dependent refractive index.Nonlinear fiber optics focuses on nonlinear optical phenomena occurring within optical fibers. The field began in 1961 with the use of a ruby laser to generate second-harmonic radiation in crystals, but the use of optical fibers as a nonlinear medium became feasible after 1970 when fiber losses were reduced to below 20 dB/km. Early studies in 1972 explored stimulated Raman and Brillouin scatterings in single-mode fibers, followed by other nonlinear effects like self- and cross-phase modulation and four-wave mixing. By 1989, the field had advanced enough to warrant a dedicated book, and it has continued to grow significantly in the 1990s, driven by advancements in high-capacity fiber-optic communication systems. The chapter highlights the unique properties of optical fibers that make them useful for nonlinear optics, particularly single-mode fibers. These fibers consist of a central core surrounded by a slightly lower refractive index cladding, made of fused silica with dopants to enhance or reduce the refractive index. The guiding properties of optical fibers are characterized by the dimensionless parameter \( V \), which determines the number of modes supported. Single-mode fibers have \( V < 2.405 \) and are used extensively in optical communications. The chapter also discusses fiber nonlinearities, which arise from the third-order susceptibility \(\chi^{(3)}\). Key nonlinear effects include third-harmonic generation, four-wave mixing (FWM), and nonlinear refraction, with nonlinear refraction being the most significant due to its role in the intensity-dependent refractive index.