Fiber lasers offer several clear advantages over solid-state systems: compact design, thermal management, minimal alignment, spatial beam quality and low cost. Consequently, fiber systems have become a valued option for applications requiring continuous-wave or long-pulse operation. However, for pulsed operation the benefits of fiber come at the cost of tighter confinement of the light, leading to the accumulation of nonlinear optical effects which can rapidly degrade the pulse. For this reason, the performance of mode-locked fiber lasers has until recently lagged behind that of their solid-state counterparts. Nonetheless, recent developments in managing nonlinearity have led to mode-locked fiber systems with performance that directly competes with solid-state systems. The aim of this thesis is to investigate the ultrashort pulse propagation physics which helps to render the nonlinear limitations of fiber systems obsolete. From the development of dissipative soliton mode-locking, which allows for an order of magnitude increase in pulse energies, to mode-locking based on self-similar pulse evolution which allows for the shortest pulses from a fiber laser to date, this thesis covers recent significant developments in laser mode-locking in systems featuring normal group-velocity dispersion. In addition, preceding pulse evolutions which were investigated experimentally, such as lasers based on self-similar propagation in a passive fiber and so-called "wave-breaking free" lasers are analyzed numerically and integrated theoretically with recent developments. Finally, several notable future directions in fiber laser research are identified and a new technique for the possible generation of ever-higher performance mode-locked fiber lasers is explored.