The rapid shift toward sustainable energy systems is reshaping the global power sector. As renewable energy sources such as solar, wind, and hydro continue to grow, their intermittent nature poses significant challenges to grid stability and reliability. This paper explores the engineering of next-generation energy systems, with a focus on renewable integration, grid flexibility, and advanced storage technologies. Renewable energy integration requires innovations in forecasting, transmission, and hybrid system design to address variability. Grid flexibility—enabled through smart grids, demand response, and distributed energy resources—plays a critical role in balancing supply and demand in real time. Advanced storage technologies, including lithium-ion batteries, flow batteries, compressed air, and hydrogen-based solutions, provide essential capacity for ensuring resilience and reliability. The study highlights technical, economic, and policy challenges while emphasizing opportunities for decarbonization, energy equity, and global sustainability. Together, these elements form the backbone of a future-proof, low-carbon energy infrastructure capable of meeting rising demands while addressing the imperatives of climate change.