A Forward observer (FO) is an artillery specialist, knowledgeable in the characteristics of artillery howitzers, medium mortars and unguided rockets; guided missiles that can support troops; and naval guns used for shore bombardment. FOs may operate as teams, with a combination of team leader, radio operator, laser designator or other marking specialist, etc. The FO team is responsible to the supported ground organization for the proper employment of fire support. They may be attached to foot-mobile special reconnaissance teams or to conventional combat units.
The FO may be on the ground with the supported troops, or in a specialized vehicle. In modern warfare, they are part of the larger system of controlling close support to ground forces.
An equivalent person that coordinates air support is called a forward air controller .
Some military organizations combine air and artillery fire support, which will tend to increase as both use precision guided munitions based on similar guidance. The U.S. Marine Corps, for example, has ANGLICO (Air Naval Gunfire Liaison Company) units, which can be assigned as teams or full companies to U.S. or foreign combat units that need specialists in artillery, naval fies, and air support. 
- 4th Anglico 1st Platoon, USMC, A little history