摘要： Multimode fibers were first used for nonlinear optics during the 1970s because most optical fibers available at that time supported multiple modes. The situation changed in the 1980s when single-mode fibers were commercialized in view of their telecommunication applications. The interest in multimode fibers re-surfaced after 2005, partly motivated by their use for space-division multiplexing in optical communication systems. As a byproduct of this interest, the nonlinear effects in multimode fibers have been studied extensively since 2010. This chapter covers recent advances in a systematic fashion. Three types of multimode fibers are introduced in Section 14.1, where we also discuss modes supported by them. Section 14.2 extends theory of Section 2.3 to obtain a set of coupled nonlinear equations for various fiber modes. These equations are used in Sections 14.3 to discuss modulation instability and soliton formation in multimode fibers. The focus of Section 14.4 is on the intermodal nonlinear effects that transfer energy among the modes. The spatio-temporal effects are discussed in Section 14.5 with emphasis on spatial beam cleanup and supercontinuum generation through multimode fibers. Section 14.6 is devoted to the nonlinear phenomena in multicore fibers.