Hermeticity requirements for electronic devices become more important as increasingly stringent demands are made upon new systems required for DOD and NASA. This paper describes flat-packs which are composed of nonporous materials which leave two areas for possible leaks: the seal between the lid and the case, and the seals between the leads and the case. The processes for sealing the lid on the package are discussed, showing the various material combinations currently being used. Sketches and photographs show the structure of various types of packages. Seal structures are discussed showing the characteristics of the Kovar-to-glass lead seal, Kovar-to-pyroceram lead seal, and the advantages and disadvantages of each. Hermeticity data are provided and relate the calculated rate of exchange of the ambient in a given package to the measured leak rate. Methods of determining leak rates are discussed, including photographs of the helium bomb and the mass spectrometer setup technique. Conditions of the fine leak test and of the gross lead test are presented.