The present invention relates to a method and a device for monitoring a flowing medium, in particular the blood flowing in an extracorporeal blood circulation, for the presence of air, in particular micro-bubbles. A sequence of signal pulses or a continuous signal is injected into the flowing medium, and the signal pulses or continuous signal leaving the flowing medium are received. To detect micro-bubbles, a signal pattern that is characteristic of the variation over time of the received signal pulses or the continuous signal in a predetermined period of time is extracted from the signal received. The characteristic signal pattern is compared with one or more characteristic reference patterns, and the presence of air bubbles is determined if the characteristic signal pattern deviates from the characteristic reference pattern by a predetermined amount. Statistical characteristic variables, in particular the variance, are preferably determined from the signal patterns and compared with one another.