Ultrafast optical pulses have been widely used in fundamental research, medical and industrial applications. For example, ultrafast lasers are used for chemistry, optical frequency metrology, terahertz generation, spectroscopy, multi-photon microscopy, optical coherence tomography, and micro-machining, etc. Traditionally, solid-state lasers, which have been engineered for over 30 years, dominate the market. Fiber lasers, as their competitors, offer several advantages over the solid-state systems: compact size, excellent thermal management, high efficiency, diffraction-limited spatial quality and low cost. Therefore, fiber lasers are becoming more popular on the continuous-wave laser market. For pulsed operation, large net nonlinear effects due to the tight confinement of the light in the core and the long propagation distance have limited their performance. As a result, the performance of pulsed fiber lasers has lagged behind that of their solid-state counterparts. In addition, product-scale adoption of high-performance ultrafast fiber lasers in industrial application is hindered by the lack of environmental-stability. This thesis focuses on the study of pulse propagation in fiber oscillators and multimode fibers, which aims to solve the above problems. An environmentally-stable fiber laser source based on cascaded Mamyshev regeneration and the formation of parabolic pulses, which allows for at least an order of magnitude increase in peak power and 6-fold increase in nonlinear phase accumulation, is demonstrated experimentally. The outstanding performance, which is ~ 50 nJ and ~ 40 fs, has also been boosted up to the comparable level as that of the Ti:sapphire lasers. In addition, The combination of excellent performance with the environmental stability make the Mamyshev oscillator extremely attractive for applications. To further improve the laser performance, multimode fibers, which can offer much larger mode field diameter and complex spatio-temporal couplings, are studied. Remarkable phenomena such as beam clean-up and self-organized instability in graded-index multimode fibers are observed and explained. Understanding the pulse propagation in those complex systems provides a route to further energy scaling. This thesis is not just limited to the generation of high-energy, short-duration, coherent pulses. It also covers interesting nonlinear dynamics such as extreme events in the all-normal dispersion fiber oscillators. This may attract attention from researchers in nonlinear systems and oceanography. Finally, future directions are discussed.