Systems Engineering examines the key elements that drive the overall system design and optimizes those elements for overall system performance, generally based on customer priorities. Systems engineering was developed and has been refined in the area of aircraft design. It has been applied to optimize the design of command, control, communications, computers, intelligence, surveillance and reconnaissance (C4ISR) systems. It has recently been employed to optimize the design for an entire military force. Systems engineering approaches are transitioning to meet the needs of customers in the 21st century.