Abstract: An ultrasonic monitor is disclosed for monitoring movement, such as breathing of a human or animal, or low frequency vibration of civil engineering structures. The monitor may also be used in measuring fluid properties such as flow rate or viscosity. The monitor operates by periodically emitting bursts of ultrasound pulses from an emitter (1), and subsequently detecting the pulses by a receiver (6), for example after reflection from a target object (13) such as a sleeping baby. Changes in condition are determined by monitoring the changes in phase occurring between the input to the emitter (1) and the output from the receiver (6).