A memory module includes a plurality of semiconductor memory devices and a circuit board. The circuit board is electrically connected to the plurality of semiconductor memory devices, and a signal line is disposed in the outermost layer of the circuit board. An electrical reference for the signal line is provided in a layer of the circuit board that is not adjacent to the outermost layer. Accordingly, an impedance of the signal line may be increased, and signal integrity of a signal transmitted through the signal line may be improved.