Book excerpt: This book covers the selection of nanocomposite supercapacitor materials. It describes the most important criteria behind the selection of materials for the electrode, electrolytes, separator and current collectors, which comprise the key components of supercapacitors for advanced energy storage. It discusses the influence on each material on the unique electrochemical properties of nanocomposite supercapacitors with respect to their energy storage mechanism and stability under extreme and unpredictable conditions. This book is part of the Handbook of Nanocomposite Supercapacitor Materials. Supercapacitors have emerged as promising devices for electrochemical energy storage, playing an important role in energy harvesting for meeting the current demands of increasing global energy consumption. The handbook covers the materials science and engineering of nanocomposite supercapacitors, ranging from their general characteristics and performance to materials selection, design and construction. Covering both fundamentals and recent developments, this handbook serves a readership encompassing students, professionals and researchers throughout academia and industry, particularly in the fields of materials chemistry, electrochemistry, and energy storage and conversion. It is ideal as a reference work and primary resource for any introductory senior-level undergraduate or beginning graduate course covering supercapacitors.