The refrigerator includes a cabinet having a storage compartment therein, an illumination device for illuminating the interior of the storage compartment, a door hinged to the cabinet for opening and closing the storage compartment, the door having an opening formed therein and a panel assembly having a front panel provided at the front thereof, a knock sensor module including a microphone module for sensing a sound wave generated by knock vibration applied to the door and a module microcomputer for determining whether the knock vibration is a predetermined knock based on the sensed sound wave, and a controller provided separately from the module microcomputer for receiving a knock-on signal from the module microcomputer and operating the illumination device such that light is transmitted through the panel assembly, whereby the interior of the storage compartment can be seen from outside the door.