A brief history of primers begins with the percussion cap, which first appeared in the early 1800's, and proceeds through the development of primers in military and sporting cartridges after World War I. The Germans developed the first practical noncorrosive, nonmercuric (NCNM) primer about 1928. The use of lead styphnate with a tetracene sensitizer has been the basis of all NCNM primers since that time. There are three major priming systems currently used in small arms ammunition: rimfire, centerfire Boxer, and centerfire Berdan. The primer contains a mixture of substances that perform three basic functions: an initiator, which is an explosive that starts the process when the firing pin hits the primer; a sensitizer, which helps in the ignition process; and a fuel, which sustains the flame and ensures adequate time to light the powder. This article indicates the substances used in these functions. 7 references and illustrative drawings.