The present invention relates to a device for determining and/or monitoring foreign structures in a fluid or in a fluid stream, in particular blood or a bloodstream, with at least one optical monitor, at least one ultrasonic monitor and at least one signal-evaluating means, wherein the fluid can be monitored optically at least by means of the optical monitor and can be monitored ultrasonically at least by means of the ultrasonic monitor, and wherein, on the basis of the combination of the resulting monitoring signals, in particular on the basis of the comparison of the resulting monitoring signals, at least one foreign structure, in particular an air bubble and/or a solid body such as a blood clot, can be detected in the fluid by the signal-evaluating means and/or can be differentiated from at least a second foreign structure by the signal-evaluating means.
