The Aegis ballistic missile defense (BMD) program, which is carried out by the Missile Defense Agency (MDA) and the Navy, gives Navy Aegis cruisers and destroyers a capability for conducting BMD operations. Under the FY2020 budget submission, the number of BMD-capable Navy Aegis ships is projected to increase from 38 at the end of FY2018 to 59 at the end of FY2024. BMD-capable Aegis ships operate in European waters to defend Europe from potential ballistic missile attacks from countries such as Iran, and in in the Western Pacific and the Persian Gulf to provide regional defense against potential ballistic missile attacks from countries such as North Korea and Iran. The Aegis BMD program is funded mostly through MDA's budget. The Navy's budget provides additional funding for BMD-related efforts. MDA's proposed FY2020 budget requests a total of $1,784.2 million (i.e., about $1.8 billion) in procurement and research and development funding for Aegis BMD efforts, including funding for two Aegis Ashore sites in Poland and Romania. MDA's budget also includes operations and maintenance (O&M) and military construction (MilCon) funding for the Aegis BMD program. Issues for Congress regarding the Aegis BMD program include the following: * whether to approve, reject, or modify MDA's FY2020 funding procurement and research and development funding requests for the program; * required numbers of BMD-capable Aegis ships versus available numbers of BMD-capable Aegis ships; * the burden that BMD operations may be placing on the Navy's fleet of Aegis ships, and whether there are alternative ways to perform BMD missions now performed by U.S. Navy Aegis ships, such as establishing more Aegis Ashore sites; * burden sharing-how allied contributions to regional BMD capabilities and operations compare to U.S. naval contributions to overseas regional BMD capabilities and operations; * whether to convert the Aegis test facility in Hawaii into an operational land-based Aegis BMD site; * the potential for ship-based lasers, electromagnetic railguns (EMRGs), and hypervelocity projectiles (HVPs) to contribute in coming years to Navy terminal phase BMD operations and the impact this might eventually have on required numbers of ship-based BMD interceptor missiles; and * technical risk and test and evaluation issues in the Aegis BMD program.