Abstract: An embodiment of a probe includes a sensor and a support. The sensor is operable to provide an indication of a thickness of a frozen substance that has accumulated between the sensor and a cooling fin of a cooling unit, and the support is operable to hold the sensor spaced apart from the cooling fin. For example, a defrost controller may use an embodiment of such a probe to monitor an amount of frost build up on the fin or fins of a cooling unit (e.g., a refrigeration or freezer unit) so that the controller may initiate a defrost cycle only when warranted. Such a probe may be more reliable than other defrost-detection techniques, and such a defrost controller may increase the cooling and energy efficiencies of a cooling unit as compared to a cooling unit having a conventional defrost controller.