During investigations of mica capacitor failures of various equipments during the last ten years and studies to improve their reliability, many mechanisms of failure have been examined. The principal features of the mechanisms are described and illustrated, and are summarized in tabular form for quick reference. Some methods of examination and dissection of these capacitors with the minimum loss of evidence are given in detail.This work does not imply that the proportion of mica capacitors which fail in service is excessive. Some of the causes of failure would occur in other types of capacitor, perhaps to a similar extent.