A sensing system for sensing the condition of an object comprises a transducer arranged to generate pressure waves directed at the object and detection means, such as a pressure wave detector, arranged to detect cavitation or other processes in the object. The system further comprises processing means arranged to receive detection signals from the detection means, to process the detection signals to measure a signal parameter of the detection signals that varies with a parameter of the object, and may generate a sensor output that varies in response to changes in the signal parameter.