A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it. It is the dual of a voltage source, which delivers a constant voltage regardless of the current flowing through it. An ideal current source generates a current that is independent of the voltage changes across it, and it is a mathematical model that real devices can approach very closely. If the current through an ideal current source can be specified independently of any other variable in a circuit, it is called an independent current source.

In real life, no physical current source is ideal. For example, no physical current source can operate when applied to an open circuit. There are two characteristics that define a current source in real life. One is its internal resistance, and the other is its compliance voltage. The compliance voltage is the maximum voltage that the current source can supply to a load. Over a given load range, it is possible for some types of real current sources to exhibit nearly infinite internal resistance.

When analyzing and designing basic circuits, voltage sources are seen more frequently than current sources because electronic systems are usually powered by batteries or regulated direct-current power supplies, and both of these function as voltage sources. However, current sources are more commonly a means of capturing the behavior of circuit components such as light-sensitive diodes and transistors. An example of a current source used in a practical circuit is the photovoltaic cell, which is modeled as a current source (in conjunction with a few other components) and can generate a constant current flow.