A Weapon Systems Officer (WSO), nicknamed "Wizzo", is an air flight officer directly involved in all air operations and weapon systems of a military aircraft.

Historically, aircrew duties in military aircraft were highly specialised and rigid, because the relevant controls, instruments/displays, and/or weapons were concentrated in front of particular seats, panels or positions. That included two-seat variants of fighter or attack/strike aircraft (including late 20th century types such as the F-4 Phantom II, A-6 Intruder, F-111 Aardvark, F-14 Tomcat, Panavia Tornado, Su-24 Fencer and Su-30MK Flanker-C, Dassault Mirage 2000N/2000D).

From the 1970s onward an aircraft with two-member crews, such as the F-15E Strike Eagle, F/A-18F Super Hornet or Su-34 Fullback and Dassault Rafale B have often featured programmable multi-function displays. These programs allow roles to be more flexible than previous generation aircraft. Multiple crew members can be responsible for detecting, targeting and engaging air-to-air or ground targets, communications, datalinks and/or defensive systems. Roles can be customized based on experience, expertise, workload, tactics, and weapons being employed. Pilots usually retain the responsibility for flying the aircraft in tactical situations. However, the crews of dedicated bomber aircraft usually retain distinct, rigidly-defined and conventional roles.