An apparatus for detecting blood clots in a blood stream of an extracorporeal blood circuit includes an optical sensor, an ultrasound sensor, and a signal evaluator. The apparatus is configured such that the blood is optically monitored by the optical sensor and ultrasonically monitored by the ultrasound sensor. The signal evaluator is configured such that the blood clots are detected by a comparison of the monitoring signals of the optical sensor and the ultrasound sensor.
