Maintaining tissue perfusion, thus, obtaining optimal oxygen delivery to the tissue is the final goal of hemodynamic monitoring. Cardiac output is the most valuable parameter among the hemodynamic variables. Although there is no true reference for the cardiac output monitoring, thermodilution cardiac output remains the current clinical standard for measuring cardiac output. New cardiac output techniques are compared with thermodilution method in accuracy and relevance, although thermodilution cardiac output monitoring has been shown to increase the morbidity and mortality, and be inaccurate in several clinical settings. New techniques of cardiac output monitoring have been introduced to increase the safety, convenience, accuracy and reproducibility. Especially, to decrease the time-delay of therapeutic intervention, continuous monitoring is preferred. Among these, partial CO2 rebreathing technique based on Fick principle and esophageal Doppler technique are optimal for intraoperative and intensive care settings during mechanical ventilation. They are non- or minimally invasive methods and don't need that much extra training to operate the machines. Esophageal Doppler monitoring provides further data on preload and contractility, but partial CO2 rebreathing technique does not. In case of cardiac or pulmonary dysfunction, cardiac output monitoring technique which could give more information about preload is beneficial. In cardiac surgery, transesophageal echocardiogram is getting to substitute for pulmonary artery catheter for cardiac output monitoring. For cardiac output monitoring, less invasive, more accurate, and continuous technique is preferred, but it should be chosen according to the clinical setting such as type of operation and cardiac and/or pulmonary function of the patient. (Korean J Anesthesiol 2004; 46: 1～9)