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 FY2019 budget submission, the number of BMD-capable Aegis ships is scheduled to be 41 at the end of FY2019 and 57 at the end of FY2023. Two Japan-homeported Navy BMD-capable Aegis destroyers included in the above figures-the Fitzgerald (DDG-62) and the John S McCain (DDG-56)-were seriously damaged in collisions with merchant ships in waters off the coasts of Japan and Singapore in June 2017 and August 2017, respectively, and are currently being repaired. The temporary loss of these two BMD-capable ships reinforced, at the margin, concerns among some observers about required numbers of BMD-capable Aegis ships versus available numbers of BMD-capable Aegis ships, particularly for performing BMD operations in the Western Pacific. Under the European Phased Adaptive Approach (EPAA) for European BMD operations, BMD-capable Aegis ships are operating in European waters to defend Europe from potential ballistic missile attacks from countries such as Iran. BMD-capable Aegis ships also operate 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 FY2019 budget requests a total of $1,711.8 million in procurement and research and development funding for Aegis BMD efforts, including funding for two Aegis Ashore sites in Poland and Romania that are to be part of the EPAA. 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 FY2019 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 European naval contributions to European BMD capabilities and operations compare to U.S. naval contributions to European BMD capabilities and operations; 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.