Electret theory is briefly summarized and discussed. The electret strength as usually measured is shown to depend on the measuring method, the electret thickness, and the storage conditions. This is caused by charge leakage and sparking. The maximum charge density as a function of electret thickness is calculated; the thicker the electret the lower the maximum charge density. Our electrets are thin (5 &mgr;m) polymer membranes, provided with evaporated gold electrodes. They do not have to be stored in a short‐circuited condition.