This book examines the latest research in carbon nanotube and nanocomposite applications. In a supercapacitor, the surface area of the electrodes accessible to the solvated electrolyte ions determines the capacitance; therefore, carbon materials with improved surface area may increase the energy density of supercapacitors. Two materials being studied for this are carbon nanotubes (CNTs) and graphene. Since all of their atoms are surface atoms, extremely large surface areas per mass may be obtainable. CNTs also have inherently high conductivity, which should increase the power density of these capacitors. To date, CNT supercapacitors have only matched state-of-the-art activated carbon supercapacitor performance. Factors limiting CNT supercapacitors are explored and future prospects are considered.