<p>With the rising global energy demand and the urgent need for sustainable solutions, waste heat recovery (WHR) has gained significant attention to improve energy efficiency and reduce environmental impact. A substantial portion of industrial, transportation, and commercial energy is lost as waste heat, which, if effectively recovered, can enhance efficiency, lower operational costs, and reduce greenhouse gas emissions. This review comprehensively explores WHR sources, methodologies, and applications across various sectors, including industrial processes, power generation, transportation, and commercial buildings. Technologies such as heat exchangers, thermal energy storage, Organic Rankine Cycle (ORC), thermoelectric generators, and supercritical CO₂ systems are discussed alongside emerging advancements in nanomaterials, artificial intelligence (AI), and Internet of Things (IoT) integration for system optimisation. Additionally, this study highlights a case of WHR in refrigeration systems, where waste heat recovery provides both cooling and heating benefits. Two novel efficiency indicators, overhead-coefficients of performance (COP) and coefficient of savings, are introduced to assess the feasibility of using waste heat for district heating networks (DHN). A case study from a food logistics centre in Germany demonstrates how waste heat, previously released into the environment, can be repurposed for sustainable residential heating. The findings emphasise the importance of selecting optimal operating temperatures to balance efficiency and energy consumption. Furthermore, economic and environmental aspects, policy incentives, and technological challenges in WHR implementation are critically analysed. This study provides valuable insights for researchers, engineers, and policymakers, reinforcing the role of WHR in achieving energy sustainability and carbon reduction goals.</p>