Electret materials are quite common in nature. Quartz and other forms of silicon dioxide, for example, are naturally occurring electrets. Today, most electrets are made from synthetic polymers, e.g. fluoropolymers, polypropylene, polyethyleneterephthalate (PET), etc. Real-charge electrets contain either positive or negative excess charges or both, while oriented-dipole electrets contain oriented dipoles. The quasi-permanent internal or external electric fields created by electrets can be exploited in various applications.