Zonal isolation is a fundamental concept and engineering challenge in subsurface operations, focusing on preventing unwanted fluid or gas migration between distinct geological formations or operational zones within a wellbore. This research area investigates the principles, material properties, placement methodologies, and long-term performance of engineered barriers—such as cement sheaths, plugs, or packers—under complex downhole conditions to ensure wellbore integrity, optimize resource extraction or injection, and mitigate environmental risks.