About 40 percent of unauthorized immigrants entered the country legally but remained after their legal status expired. Entry-exit systems were originally conceived as a tool to identify overstays—in other words, to track when foreign nationals enter the country and whether they leave in accordance with the terms of their admission. A complete entry-exit system would track all arrivals and departures at all land, air, and sea ports of entry (POE).
Over the past 18 years, Congress passed several laws requiring the implementation of an entry-exit system that does not inhibit the free flow of legitimate commerce and travel. After the events of September 11, 2001, an entry-exit system was also viewed as a national security asset. In response, Congress added biometric identifiers (such as fingerprints) to the statutory requirements, in addition to previously-required biographic identifiers (such as names and birthdates).
The full report PDF can be found here: BPC Immigration Entry-Exit System Progress, Challenges, and Outlook