A non-contact apparatus is provided for monitoring the height of contents of a moving container, such as a beverage can 24 on a conveyor 14. The apparatus comprises non-contact ultrasonic wave generation means comprising a first electromagnetic acoustic transducer 16 disposed to one side of a container 24 being monitored and adapted to produce an ultrasonic wave pulse in the container, and non-contact ultrasonic wave detector means comprising a second electromagnetic acoustic transducer 18 adapted to detect an ultrasonic signal and to generate a detection signal dependent upon the ultrasonic signal which is detected, the detector means being disposed at a different position to the side of a container 24 from the generation means. The apparatus further including signal processing means 20 adapted to process the detection signal generated by the ultrasonic wave detector means using the difference in transit times for two waves to provide an indication of the level of the contents in the container 24.