To achieve the goals of resuscitation in critically ill patients, a thorough understanding of the techniques available to measure cardiac output is important. Recently the pulmonary artery catheter has fallen out of favor because of concerns of safety and a lack of efficacy. Newer less invasive techniques since have been developed and are gaining popularity. But is important to remember that the ability of these techniques to improve outcome has yet to be demonstrated, and one should apply caution in how they are used until their use in algorithmic treatment approaches have been shown to improve outcome. This article discusses the invasive and noninvasive techniques to assess cardiac output.