Thermal energy storage (TES) technologies are currently employed to store waste/excess heat that can be released when and where needed, thereby filling the gap between energy demand and availability. Among the most widely used materials for TES are organic phase change materials (PCMs), such as paraffins, poly(ethylene glycol)s, and fatty acids and alcohols, which absorb a large amount of latent heat at a nearly constant temperature and are thus suitable to keep the temperature in a specific range or under certain threshold, useful for example in the thermal management (TM) of electronic devices. The incorporation of these organic PCMs in polymer composites results in multifunctional materials suitable in those applications requiring high specific mechanical properties and heat management (e.g., electric vehicles). Although the PCMs generally decrease the mechanical properties of the host structural composites, such multifunctional composite can still be beneficial in terms of mass saving, compared to two monofunctional units performing the structural and heat management functions individually.This book briefly introduces the concept of TES and PCMs, with a special focus on organic solid-liquid PCMs, their confinement methods and their TM applications al low-medium temperatures (0 °C–100 °C). It then investigates the approach of embedding TES and TM functionalities in structural materials, through the development of multifunctional polymer composites that could find applications where weight saving and temperature management are equally important. The concept of structural TES composite will be presented through the description of some case studies.