This thesis investigates the dynamics of passively mode-locked semiconductor lasers, with a focus on the influence of optical feedback on the noise characteristics. The results presented here are important for improving the performance of passively mode-locked semiconductor lasers and, at the same time, are relevant for understanding delay-systems in general. The semi-analytic results developed are applicable to a broad range of oscillatory systems with time-delayed feedback, making the thesis of relevance to various scientific communities. Passively mode-locked lasers can produce pulse trains and have applications in the contexts of optical clocking, microscopy and optical data communication, among others. Using a system of delay differential equations to model these devices, a combination of numerical and semi-analytic methods is developed and used to characterize this system.