Thermal energy storage (TES) systems are examined from the perspectives of energy, exergy, environmental impact, sustainability and economics. Reductions possible through TES in energy use and both pollution levels and environmental impact are described and highlighted with an illustrative example. The importance of using exergy analysis to obtain more realistic and meaningful assessments than the conventional energy analysis of the efficiency and performance of TES systems is demonstrated. The results indicate that exergetically efficient TES can play a significant role in meeting society's preferences for more efficient, environmentally benign, sustainable and economic energy use in various sectors, and appears to be an appropriate technology for addressing the mismatches that often occur between the times of energy supply and demand.Units: SI