A resistor is an electronic component that restricts the flow of electric current. It is used to reduce voltage levels, limit current flow, and to divide voltages. Resistors come in a variety of sizes, shapes, and materials in order to meet a variety of power and environment needs. The resistance of a resistor can be calculated, and is determined by the material of which it is made, as well as its physical dimensions. Resistors are often used in circuits with capacitors and transistors in order to maintain voltage levels and control the amount of current in a circuit.