The fleet of manned aircraft that accomplishes the Department of Defense's (DOD's) Command and Control (C2) and Intelligence, Surveillance, and Reconnaissance (ISR) missions for the joint military community (E-8, E-3, RC-135, WC-135, OC-135, and E-6) is primarily based on Boeing 707 aircraft procured from the 1960s to the early 1990s. As the age of these legacy C2ISR aircraft increases, understanding the Air Force and Navy modernization and recapitalization plans is likely important for Congress. The central issue addressed in this report is the benefits and drawbacks of continued sustainment and modernization of these aircraft compared to funding for recapitalization and transfer of these missions to new aircraft. This report examines the Air Force's and Navy's current sustainment, modernization, and recapitalization efforts for these Boeing 707-based aircraft. Without sufficient sustainment and modernization funding, many analysts argue the legacy C2ISR fleet is likely to become a decrepit force ill-suited to the potential challenges posed by future adversaries. The physical wear-and-tear on these aircraft from the demands of their already lengthy service lives leads aging airframe structures to need reinforcement, engines to require maintenance, and computer and electronic components to face obsolescence. Even if corrosion, metal fatigue, and parts obsolescence do not take their toll on the fleet, military analysts point out that potential adversaries are acquiring advanced, anti-access/area-denial (A2/AD) weapon systems that would make it harder for the legacy C2ISR aircraft to get close to the battle. This report addresses potential congressional oversight and appropriations concerns for the sustainment, modernization, and recapitalization of the DOD's Boeing 707-based legacy C2ISR aircraft fleet. It does not address options for recapitalization currently being offered by industry to other countries. Congress has the authority to approve, reject, or modify Air Force and Navy funding requests for C2ISR aircraft sustainment, modernization, and recapitalization, as well as maintain oversight of the nation's C2ISR requirements and capabilities. Congress's decisions on appropriations for the C2ISR force could impact the nation's C2ISR capabilities and have additional consequences for the U.S. aerospace industry. A key issue for Congress is whether to continue providing sustainment, modernization, and/or recapitalization funding for DOD's Boeing 707-based legacy C2ISR aircraft fleet, and if so, at what levels. Pertinent to the discussion is the potential for a shortfall in the nation's C2ISR capabilities if Congress or DOD chooses to minimize funding for sustainment and upgrades that would keep the weapon systems viable until they are recapitalized. Another issue for Congress is DOD, Air Force, and Navy priorities for sustainment, modernization, and recapitalization efforts. An important question is whether development of a C2ISR replacement aircraft can or should be further delayed by increasing funding for legacy C2ISR aircraft sustainment and modernization. Additionally, potentially shifting some of the legacy C2ISR missions to remotely piloted aircraft may affect recapitalization efforts. Congress's decisions on these issues could also have implications for any potential future base realignment and closure (BRAC) decisions as well as impact the U.S. aircraft manufacturing industrial base. Ultimately, the priority the DOD places on legacy C2ISR sustainment, modernization, and recapitalization, and any decisions considered by Congress, could have potential consequences for future national defense strategies and on U.S. C2ISR capabilities, given the pivotal role C2ISR plays in implementing those strategies. A note on sources: As small fleets of specialized aircraft, relatively little has been written on the programs discussed in this paper. Description of each therefore relies on Air Force fact sheets, which are used to introduce each aircraft before discussing the supporting budget items. In 1952, the Boeing Company began development of the Model 367-80 prototype aircraft, known as "Dash 80." That prototype led to the military KC-135 tanker and the commercial 707 aircraft, which shared the basic design of the Dash 80 but were very different airplanes, neither being a derivative of the other. A total of 820 C/KC-135s were built through 1966, 732 as tankers and 88 modified for special purposes, including cargo carriers, reconnaissance airplanes, Strategic Air Command airborne command posts, and transports for high-ranking government officials. Boeing delivered 856 Model 707s between 1957 and 1994, with 725 delivered between 1957 and 1978 for commercial use. Most civil 707s left in service after 2007 were converted to freighters, while a number were used as corporate transports. The Department of Defense's current C2ISR aircraft fleet based on the Boeing 707 includes 89 operational aircraft. Table 1 lists the types of aircraft and their average age. Additional information on each aircraft type, the current sustainment and modernization efforts, previous recapitalization efforts (if applicable), and current recapitalization efforts follows the table. Despite the advanced age of the Boeing 707-based DOD C2ISR aircraft fleet, aircraft availability remain at levels sufficient to sustain current operational requirements. Table 2 lists the percentage of time aircraft were available, known as mission-capable rates, for the past five years. The E-8C JSTARS is an airborne battle management, command and control, intelligence, surveillance, and reconnaissance platform. Its primary mission is to provide theater ground and air commanders with ground surveillance to support targeting and attack operations. The E-8C is a modified Boeing 707-320 commercial airframe extensively remanufactured and modified with radar, communications, operations, and control subsystems required to perform its mission. The most prominent external feature is a 27-foot long, canoe-shaped radome under the forward fuselage that houses a 24-foot long, side-looking phased array antenna. The antenna can tilt to either side of the aircraft where it can develop a 120-degree field of view covering nearly 19,305 square miles, and is capable of detecting targets more than 155 miles away. (In short, the JSTARS' sensors detect and monitor vehicles and other ground-based objects over an area roughly twice the size of New Jersey.) The radar also has limited capability to detect helicopters, rotating antennas, and low, slow moving fixed wing aircraft. The information is relayed in near real time to ground command, control, communications, computers, and intelligence nodes. The JSTARS concept evolved from U.S. Army and Air Force programs to develop, detect, locate, and attack enemy armor at ranges beyond the forward area of troops. The ability of airborne side-looking radar to detect slow-moving and fixed targets on the ground was demonstrated in the Air Force's Pave Mover program in 1982. JSTARS combines technologies developed in Pave Mover and the Army's Stand-Off Target Acquisition System program. A developmental contract for the E-8 JSTARS was awarded to Grumman Corporation in September 1985. Original plans called for 10 conversion aircraft. The plan later changed to 22 newly-built aircraft and subsequently changed again to 20 aircraft before a final total of 17 operational aircraft emerged. The production E-8s were originally planned to be new aircraft based on the U.S. Navy's E-6 Hermes aircraft (a Boeing 707-320 aircraft with more powerful engines), however, in October 1989 Boeing announced the end of production of the 707 airframe upon completion of British and French airborne warning and control system aircraft in 1991. As a result, the E-8C aircraft are retrofitted Boeing 707 aircraft acquired from other sources. The first two test aircraft were former Boeing 707-328 commercial airliners from American Airlines and Qantas Airlines. The first E-8C entered operational service in late 1995. The E-8C aircraft fleet averages 13.6 years of age. This is based on when the aircraft became E-8C JSTARS aircraft, not when they were originally manufactured by Boeing as 707-300s in the 1960s. The 116 th Air Control Wing, a Georgia Air National Guard unit, and the 461 st Air Control Wing, an associate active-duty Air Combat Command unit, operate 16 operational E-3C JSTARS at Robins Air Force Base, Georgia. With the Air Force's decision to recapitalize the E-8C JSTARS fleet beginning in FY2015, the Administration's FY2015 budget request did not include modernization funding for the E-8C JSTARS. To fund E-8C JSTARS recapitalization, the Air Force planned (as part of the FY2015 Budget Request) to divest the E-8C test capability, including the T-3 test aircraft, and begin the retirement of five operational E-8C JSTARS aircraft. As part of the National Defense Authorization Act for FY2015, Congress prohibited the Air Force from taking any action to retire or to prepare to retire operational JSTARS aircraft until 30 days after the Secretary of the Air Force submits a report to the congressional defense committees providing an update of the results of the analysis of alternatives for recapitalizing the JSTARS capability and an assessment of the cost and schedule of developing and fielding a new aircraft and radar system to replace the current JSTARS aircraft. The Administration's FY2016 budget request delays the divestiture of five operational E-8C JSTARS aircraft from FY2016 to FY2019 and restores funding to complete one modernization program for those five operational aircraft that was in progress for the other 11 operational aircraft prior to FY2015.