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The review begins with a very brief history of electrets and of piezo and pyroelectricity in nonuniform space charge electrets. The development of cellular propylene as an electret-transducer material since the mid-eighties is described, and the accompanying research into the origins of (quasi-)ferro-, pyro- and piezoelectric effects in voided polymer electrets is discussed in some detail. In this context, the most relevant charging and measuring techniques for piezo and pyroelectric polymer-foam electrets are introduced. Porous fluoropolymer electrets and their investigation as single or multiple-layer piezoelectrics are considered in view of their better thermal and temporal stabilities. Finally, some proposed or demonstrated applications of voided polymer electrets are briefly mentioned.