The rapid growth of renewable energy technologies has intensified the need for efficient and sustainable energy storage systems. In this context, electrochemical energy storage devices with high performance and reliability are receiving increasing attention. Among the available technologies, supercapacitors have gained significant interest because of their exceptional power density, fast charging-discharging capability, long operational lifetime, and environmentally friendly nature. This chapter provides an overview of the fundamental principles governing supercapacitor technology. It discusses the major categories of supercapacitors, including electrochemical double-layer capacitors (EDLCs), pseudocapacitors, and hybrid supercapacitors. The mechanisms responsible for energy storage in these systems are explored in detail. These include the electrostatic charge accumulation at the electrode–electrolyte interface in EDLCs, rapid and reversible faradaic redox reactions in pseudocapacitors, and the combination of both mechanisms in hybrid devices. In addition, key electrochemical performance indicators such as specific capacitance, energy density, power density, and cycling stability are analyzed and compared across different types of supercapacitors. Through this discussion, the chapter establishes a fundamental understanding of how charge storage behavior influences device performance and highlights important considerations for the development of advanced supercapacitor systems for future energy storage applications.