See also: line of sight in gaming, referring to visibility of units.

When viewing a scene, as in optics, photography, or even hunting, the line of sight is the straight line between the observer and the target. In astronomy, when the distance between the observer and the distant objects is large, the light of sight can often be taken as the straight line between the earth and the target.

Line of sight is commonly used to refer to telecommunication links that rely on a line of sight between the transmitting antenna and the receiving antenna. Such capability is necessary for high frequency microwave links that offer relatively high bandwidth communication circuits. Typical operating frequencies are in the gigahertz frequency range where the radio path is not reflected or refracted to any great extent. Occasionally this term is mis-written as "line of site."

Typical transmission path lengths are of the order of forty miles (60 km) but the height of the antennae and intervening terrain have significant influence. Many links are now being upgraded to fibre optic cable.

Line of sight is also be necessary for optical transmission systems which can be used for short distances, between two high buildings, where a cable link might be very long.