Cardiac output is considered an important parameter when assessing the cardiovascular status of a critically ill patient. Both non-invasive (e.g. bioimpedance, echocardiography) and invasive methods (Swan Ganz catheter) have been used to measure cardiac output. The ultrasonic cardiac output monitoring device provides a new method of non-invasively assessing cardiac output in various clinical settings. The ultrasonic cardiac output monitoring device was introduced clinically in 2001, and appears to be a promising adjunct in the assessment of the cardiovascular state in a variety of patient cohorts. In this short review article, we will introduce this new technique, discuss the required skills and compare it with methods already in use. In particular, a critical comparison with the ‘gold standard’, the invasive measurement of cardiac output with the pulmonary artery catheter, will be given.