This textbook provides a comprehensive and practical introduction to renewable energy systems, covering the scientific principles, engineering design, operational analysis, and sustainability implications of modern energy technologies. It explores major renewable resources including solar, wind, hydropower, biomass, geothermal, and ocean energy systems, with detailed explanations of system components, energy conversion processes, performance analysis, and grid integration challenges.Designed for students, researchers, and practicing engineers, the book combines theoretical foundations with worked examples, quantitative models, design procedures, and problem-solving exercises to enhance understanding and practical application. Key topics such as climate change, energy transition, energy policies, emission reduction models, and renewable energy economics are also discussed within a global and developing-country context.With its clear explanations, engineering-oriented approach, and extensive calculation-based illustrations, this textbook serves as an essential resource for courses and professional studies in renewable energy engineering and sustainable power systems.