The number of unaccompanied alien children (UAC, unaccompanied children) apprehended at the Southwest border between U.S. ports of entry while attempting to enter the United States without authorization has increased substantially in recent years: from 16,067 in FY2011 to 24,481 in FY2012 to 38,759 in FY2013. In FY2014, the Department of Homeland Security (DHS), Customs and Border Protection (CBP) apprehended 68,541 UAC, a record at that time. Since FY2014, UAC apprehensions have fluctuated considerably, declining to 39,970 in FY2015, increasing to 59,692 in FY2016, declining to 41,435 in FY2017, and increasing to 50,036 in FY2018. In the first 11 months of FY2019, they reached 72,873, a level that now exceeds the FY2014 peak. UAC are defined in statute as children under age 18 who lack lawful immigration status in the United States, and who are either without a parent or legal guardian in the United States, or without a parent or legal guardian in the United States who is available to provide care and physical custody. Two statutes and a legal settlement directly affect U.S. policy on the treatment and administrative processing of UAC: the Trafficking Victims Protection Reauthorization Act of 2008 (P.L. 110-457), the Homeland Security Act of 2002 (P.L. 107-296), and the Flores Settlement Agreement of 1997 (Flores). Agencies in the Departments of Homeland Security (DHS) and Health and Human Services (HHS) share responsibility for the processing, treatment, and placement of unaccompanied children. DHS's Customs and Border Protection (CBP) apprehends and detains UAC arrested at the border. DHS's Immigration and Customs Enforcement (ICE) handles custody transfer and repatriation responsibilities, apprehends UAC in the interior of the country, and represents the government in removal proceedings. HHS's Office of Refugee Resettlement (ORR) coordinates and implements the care and placement of UAC in appropriate custodial settings. In FY2009, children from Mexico accounted for 82% of the 19,688 UAC apprehensions at the Southwest border, while those from the "Northern Triangle" countries of El Salvador, Guatemala, and Honduras accounted for 17%. By the first 10 months of FY2019, the proportions had reversed, with Mexican nationals comprising 12% of the 69,157 UAC apprehensions at the border and the three Central American countries comprising 85%. The Obama and Trump Administrations, as well as Congress, have taken several steps since 2014 to respond to UAC migrants. During 2014, when UAC apprehensions surged far beyond previous levels, the Obama Administration developed a working group to coordinate the efforts of relevant agencies. It also opened temporary "influx" shelters and holding facilities to accommodate the large number of UAC apprehended at the border, initiated programs to address root causes of child migration in Central America, and requested funding from Congress to deal with the crisis. In turn, Congress considered supplemental appropriations for FY2014 and provided increased funding for UAC-related activities in ORR and DHS appropriations for subsequent fiscal years. The Trump Administration, facing relatively high levels of UAC apprehensions, as well as record high levels of family unit apprehensions, has used temporary influx shelter housing for unaccompanied minors while also attempting to reduce both the flow of migrants illegally crossing the Southwest border and limit who can apply for asylum. The Administration has implemented a biometric and biographic information-sharing agreement between ORR and DHS. During six weeks in 2018, it implemented a "zero tolerance" policy targeting illegal border crossing that effectively separated thousands of children from their parents and reclassified them as UAC. The Administration has also proposed regulations to replace Flores and allow ICE to detain parents and children together indefinitely.