High-precision optical systems are essential in fields ranging from autonomous manufacturing to astronomical imaging. However, their performance is often limited by optical aberrations arising from misalignments, environmental disturbances, and internal system imperfections. This thesis investigates the principles and practical implementations of wavefront sensing and control — primarily focal-plane techniques — to enhance optical system performance in two key domains: automated alignment and high-contrast exoplanet imaging with the Gemini Planet Imager. In the context of automated optical alignment, we developed and tested a novel alignment framework for a double off-axis parabolic mirror system — an inherently challenging configuration due to its sensitivity to aberrations and nonlinear misalignment dynamics. Leveraging focal-plane wavefront sensing to eliminate non-common-path errors, and applying model-based nonlinear state estimation and control, we demonstrate micron-level alignment precision. A key discovery was the multi-state coupling effect, in which different misalignment states interact in ways that interfere with estimator performance. This phenomenon was further analyzed through observability analysis. For high-contrast imaging, we contributed to the ongoing upgrade of the Gemini Planet Imager to improve its capability to directly image and characterize exoplanets. In the adaptive optics control pipeline, we established design tools based on system stability metrics and error transfer function analysis. We further implemented a focal-plane wavefront control algorithm based on classical speckle nulling to effectively mitigate speckle noise caused by post-coronagraph aberrations and internal optical imperfections — a historical performance limitation of GPI. These results represent a significant improvement over GPI’s baseline and will be integrated into its next-generation upgrade to enable detection of lower-mass exoplanets at smaller inner working angles. By combining optical engineering, estimation theory, and control design, this thesis highlights the versatility and critical importance of focal-plane wavefront control in advancing both laboratory-based and astronomical optical systems.