The development of nonlinear optics has benefited not only the fundamental sciences but also many applications such as laser frequency conversion, optical switching, and biomedical imaging. Optical fiber, as a type of waveguide that is cost-effective, optical-power-scalable, robust, and compact, would be an intriguing platform for nonlinear optics. For a long time, people have focused on nonlinear optical dynamics in single-mode fiber. Although the spatial evolutions are limited, the single-mode system is still complicated enough and there are many interesting nonlinear phenomena being revealed such as optical soliton propagation, stimulated Raman scattering, and stimulated Brillouin scattering. Multimode optical fibers support multiple transverse modes in the waveguide. Nonlinear propagation of short light pulses results in coupling of spatial, spectral and temporal properties of the field. It is only in the last decade that people have developed efficient conceptual and simulation tools to handle such complicated evolutions. In return, exploration of the nonlinear spatiotemporal optics in multimode fiber offers a flexible and cost-efficient platform for exploring scientifically interesting nonlinear spatiotemporal phenomena. Multimode fibers also offer many possibilities for applications, such as scaling the power of short-pulse lasers and amplifiers, and optical communication with space-division multiplexing. This thesis focuses on the study of nonlinear spatiotemporal propagation in the anomalous-dispersion regime of multimode fibers. The fundamental knowledge for multimode nonlinear study is shown in Chapter 1. Kerr beam self-cleaning (KBSC) is a non-dissipative process in which optical power flows from higher-order modes to the lowest mode in nonlinear propagation. In Chapter 2, We focus on the KBSC of femtosecond pulsed beams in the anomalous dispersion regime in multimode graded-index fiber. Our results show modest beam-cleaning and strong pulse compression by a factor of ~10. We believe the results will further the understanding of KBSC and offer the potential application in high-energy pulse compression. Lasers based on soliton-like pulse-shaping dominate ultrashort pulse generation. Spatiotemporal mode-locking with anomalous dispersion can be regarded as the multimode analog of soliton lasers based on single-mode fiber, but with richer nonlinear dynamics owing to intermodal interactions. In Chapter 3, we present numerical and experimental observations of spatiotemporal mode-locking with anomalous dispersion. The results add new understanding of spatiotemporal mode-locking and illustrate the issues that must be addressed to create multimode soliton lasers. In multimode fibers, anomalous-dispersion pulse propagation involves both spatial and temporal degrees of freedom, leading to complex and intriguing soliton features that are not well understood. In Chapter 4, We report theoretical and experimental studies of highly-multimode solitons in step-index fiber. These are the first solitons to exhibit speckled intensity profiles. Numerical simulations agree reasonably with the experimental results. The results will help provide a framework for a variety of multimode nonlinear phenomena and may be relevant to applications such as space-division multiplexing communication, and imaging. Finally, in Chapter 5, some future directions are discussed.