Publisher Summary This chapter focuses on capacitance. The chapter describes constant voltage and constant current sources. It reviews that an ideal constant voltage source has zero internal resistance and an ideal constant current source has infinite internal resistance. Practical energy sources are nearly always approximations to constant voltage devices, although sources that approximate constant current devices also occur. Either type of energy source may readily be approximated to any required degree of accuracy by suitable electronic circuitry. The chapter highlights the storage of energy in a capacitor. The dielectric constant or relative permittivity of a substance is the factor by which the capacitance is increased when that substance is substituted for a vacuum between the plates. Capacitors as used in electronic equipment, charge and discharge of capacitor-resistor combination, capacitor leakage testers, alternating voltage applied to a capacitor and alternating voltage applied to a capacitor, and resistor in series are also discussed.