Book excerpt: PERSEREC developed an unclassified database of Americans involved in espionage against the United States since 1945, and in 1992 published a report on it entitled Americans Who Spied Against Their Country Since World War II. The goal of the original project was to analyze the cases in terms of themes and trends that would further our understanding of the phenomenon of espionage. Since 1992, further instances of espionage by American citizens have come to light, and we have continued to enter them into an espionage database. An updated analysis incorporating recent cases seemed useful. In this update we redefined the parameter of the database and of the report to include only Cold War cases, and we created a separate database with cases from the era of World War II. The date of the beginning of the Cold War is debatable, so we chose a starting point in the late 1940s for the database discussed in this report; this allowed us to include cases of espionage from the late 1940s that resembled those in the 1950s, and to exclude cases that were more like those in the war years. This study covers the time period 1947 through 2001. Our databases continue to be based on open source materials. In the espionage database we have included 150 individuals who were convicted or prosecuted for espionage or for attempting to commit espionage, or for whom clear evidence of espionage exists, even though for various reasons they were not convicted. This latter category includes people who defected before they were prosecuted, those who died or committed suicide before they could be prosecuted, and those who plea-bargained for lesser charges or who were given immunity from prosecution. This unclassified study, like its predecessor in 1992, deals with individuals whose names and cases surfaced in open source materials. It is impossible to know how many more spies have been identified but whose cases remain classified, how many were identified but not prosecuted (often to prevent the release of information in open court), how many spied in the past and were not identified, or how many are spying at present and remain unidentified. Unfortunately for the student of espionage, government records include more cases of espionage than are described here, but access to these is classified and restricted to the relatively small, cleared community. This database represents the information that is publicly available; it is an open source subset of the larger universe of all espionage committed by American citizens.