Navy Aegis Ballistic Missile Defense (BMD) Program: Background and Issues for Congress [Updated October 20, 2021] [open pdf - 2MB]
From the Summary: "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. 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. MDA's FY2022 budget submission states that 'by the end of FY 2022 there will be 48 total BMDS [BMD system] capable ships requiring maintenance support.' 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 FY2021 budget requested a total of $1,647.9 million (i.e., about $1.6 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."
CRS Report for Congress, RL33745
Congressional Research Service: https://crsreports.congress.gov/