This chapter of the book "Advanced Energy Storage Devices: Basic Principles, Analytical Methods, and Rational Materials Design" provides a comprehensive overview of the fundamental principles, analytical methods, and material design strategies for advanced energy storage devices. The authors, Jilei Liu, Jin Wang, Chaohe Xu, Hao Jiang, Chunzhong Li, Lili Zhang, Jianyi Lin, and Ze Xiang Shen, highlight the significant efforts made in developing high-performance energy storage devices with nanoscale design and hybrid approaches. They discuss the blurring boundaries between electrochemical capacitors and batteries, where the same material can exhibit both capacitive and battery-like behavior depending on the electrode design and charge storage ions. The chapter begins by introducing the urgent need for renewable and sustainable energy sources, emphasizing the importance of efficient energy storage systems for various applications. It classifies energy storage technologies into mechanical, chemical, electrical, and electrochemical systems, with electrochemical energy storage (EES) systems, particularly electrochemical capacitors (ECs) and batteries, showing great potential due to their high round-trip efficiency, long cycle life, and compatibility with various chemistries. The authors then delve into the kinetic and material perspectives of the similarities and differences between ECs and batteries, discussing basic techniques and analysis methods to distinguish between capacitive and battery-like behavior. They propose guidelines for material selection, advanced electrode design, and the state-of-the-art materials used in high-performance energy storage devices. The chapter also covers the principles of energy storage in ECs, including the fundamental electrochemistry and kinetic features of ECs. It explains the charge storage mechanisms in EDLCs and pseudocapacitors, and provides detailed analyses of the redox peak difference (ΔEac), the relationship between response current and sweep rate, and the quantification of capacitive and diffusion-limited processes. Finally, the chapter discusses the optimization of pseudocapacitive electrode design, highlighting intrinsic and extrinsic pseudocapacitive materials such as TiO2 (B), α-MoO3, T-Nb2O5, and Li4Ti5O12. It emphasizes the importance of material structure, crystallinity, and morphology in determining the charge storage mechanisms and performance of these materials.This chapter of the book "Advanced Energy Storage Devices: Basic Principles, Analytical Methods, and Rational Materials Design" provides a comprehensive overview of the fundamental principles, analytical methods, and material design strategies for advanced energy storage devices. The authors, Jilei Liu, Jin Wang, Chaohe Xu, Hao Jiang, Chunzhong Li, Lili Zhang, Jianyi Lin, and Ze Xiang Shen, highlight the significant efforts made in developing high-performance energy storage devices with nanoscale design and hybrid approaches. They discuss the blurring boundaries between electrochemical capacitors and batteries, where the same material can exhibit both capacitive and battery-like behavior depending on the electrode design and charge storage ions. The chapter begins by introducing the urgent need for renewable and sustainable energy sources, emphasizing the importance of efficient energy storage systems for various applications. It classifies energy storage technologies into mechanical, chemical, electrical, and electrochemical systems, with electrochemical energy storage (EES) systems, particularly electrochemical capacitors (ECs) and batteries, showing great potential due to their high round-trip efficiency, long cycle life, and compatibility with various chemistries. The authors then delve into the kinetic and material perspectives of the similarities and differences between ECs and batteries, discussing basic techniques and analysis methods to distinguish between capacitive and battery-like behavior. They propose guidelines for material selection, advanced electrode design, and the state-of-the-art materials used in high-performance energy storage devices. The chapter also covers the principles of energy storage in ECs, including the fundamental electrochemistry and kinetic features of ECs. It explains the charge storage mechanisms in EDLCs and pseudocapacitors, and provides detailed analyses of the redox peak difference (ΔEac), the relationship between response current and sweep rate, and the quantification of capacitive and diffusion-limited processes. Finally, the chapter discusses the optimization of pseudocapacitive electrode design, highlighting intrinsic and extrinsic pseudocapacitive materials such as TiO2 (B), α-MoO3, T-Nb2O5, and Li4Ti5O12. It emphasizes the importance of material structure, crystallinity, and morphology in determining the charge storage mechanisms and performance of these materials.