This chapter examines the principles, applications, and challenges of energy efficiencyEnergy efficiency and smart gridSmart grids technologies in promoting environmental sustainabilityEnvironmental sustainability. With escalating global energy demandsEnergy demand, these innovationsInnovation in sustainability emerge as critical strategies to mitigate the environmental impactsEnvironmental impact of energy production and consumption. Energy efficiencyEnergy efficiency, often termed the “first fuel,” minimizes energy waste while enhancing service qualityQuality. Smart gridsSmart grids, characterized by advanced communication systems and real-time monitoring, optimize energy distribution and integrate renewable energyRenewable energy sources seamlessly into traditional grids. Together, they foster sustainable energy practices, reduce greenhouse gas emissionsGreenhouse gas emissions, and support the global transition to carbon neutralityCarbon neutrality. Technological advancements, such as energy-efficient appliances, smart metersSmart meters, and IoTInternet of Things (IoT)-enabled systems, are highlighted for their significant contributions to energy conservationEnergy conservation. MetricsQuantitative sustainability metrics and standards like ISO 50001ISO 50001 and Energy Star certificationEnergy star certification are discussed as tools to measure and benchmark energy efficiencyEnergy efficiency efforts. In parallel, the chapter examines smart gridSmart grids technologies, including demand responseDemand response systems, distributed energy resourcesDistributed energy resources (DERs), and blockchain-enabled energy transactions. These technologies enhance grid resilienceGrid resilience, enable real-time energy management, and facilitate the adoption of renewable energyRenewable energy, reducing dependency on fossil fuels. Case studies from smart citiesSmart cities and international initiatives illustrate successful applications and scalable models for integrating these technologies. Despite their potential, the chapter acknowledges barriers to adoption, including economic constraintsEconomic constraints, technical challenges, and cybersecurity concerns. It underscores the need for robust policy frameworks and international collaboration to overcome these obstacles. By addressing these challenges, energy efficiencyEnergy efficiency and smart gridSmart grids technologies can pave the way for a sustainable, resilient, and low-carbon energy future.