Norton's theorem holds, to illustrate in DC circuit theory terms .Any linear electrical network with voltage and current sources and only resistances can be replaced at terminals A–B by an equivalent current source Ino in parallel connection with an equivalent resistance his equivalent current Ino is the current obtained at terminals A-B of the network with terminals A-B short circuited.

This equivalent resistance Rno is the resistance obtained at terminals A-B of the network with all its voltage sources short circuited and all its current sources open circuited.

For alternating current (AC) systems the theorem can be applied to reactive impedances as well as resistances.

The Norton equivalent circuit is used to represent any network of linear sources and impedances at a given frequency.

Norton's theorem and its dual, Thévenin's theorem, are widely used for circuit analysis simplification and to study circuit's initial-condition and steady-state response.