Microembolic signals (MES) can be detected in many recipients of mechanical aortic valve prostheses by transcranial Doppler ultrasound. The nature and etiology of these MES have remained unclear for a long time. The solid and gaseous nature of MES are discussed, as well as whether or not MES may reflect artifacts. Recently, the gaseous nature of these MES has been widely established. To understand the physics of bubble formation related to mechanical heart valve prostheses, it is necessary to discuss the different types of cavitation occurring at the prostheses and the conditions leading to the degassing of blood. We describe the history of transcranial Doppler ultrasound-techniques and the current techniques in the measurement of these signals. Furthermore, the possible clinical impact of MES, as well as strategies for the design of new prostheses and surgical alternatives to diminish their load are discussed.