The benefit of any hemodynamic monitoring technique is to provide reliable and reproducible information on the cardiocirculatory status of a patient in shock. The collected values will enable the intensivist to understand the hemodynamic conditions of the patient and to make more informed treatment decisions to optimize the hemodynamic status and improve the prognosis of the patient. Hemodynamic monitoring is required to assess systemic and regional tissue perfusion as the correction of circulatory instability and tissue hypoperfusion is essential to prevent the occurrence of multiple organ failure. Resuscitation is characterized by a very close temporal relationship between monitoring, decision-making, and treatment. Indeed, making prompt, appropriate management and diagnostic and therapeutic decisions in cases of hemodynamic instability reduces mortality of critically ill patients [1]. To make treatment decisions, the intensivist has an arsenal of monitoring devices. However, before using a device, it is imperative that the intensivist has sound knowledge of the pathophysiology of shock states to identify the parameters that he/she wants to monitor. Therefore, knowing the different hemodynamic monitoring parameters is important [2]. For instance, it is now established and accepted by clinicians that fluid responsiveness is determined by monitoring dynamic indices, not static indices [3]. In addition, it is becoming increasingly common for mechanically ventilated critically ill patients to not be curarized. This condition requires clinicians to adapt their practice and to use other tactics to help assess the volume status of their patients. This is particularly true with the passive leg raising maneuver. However, the issue at hand is the overall choice of monitoring technique. In the 1970s, the only advanced hemodynamic monitoring option was the pulmonary artery catheter. The use of the pulmonary artery catheter (PAC) has been challenged in recent years, and there has been debate regarding its impact on patient survival. Conflicting results have been published [5], though the widely varying conclusions are due to patient selection, incomplete information, and differences in specific treatment protocols (or lack thereof) [6]. In light of this and by following the developments in the industry, clinicians are now in favor of using less-invasive monitoring techniques. During the past few years, different techniques have been made commercially available. These devices are diverse in concept, design, and functionality but have more or less been shown to be reliable in clinical practice. Moreover, relative to the PAC, these devices are more easily handled, which could lead to their adoption and early application for use in large populations of at-risk patients or in patients with hemodynamic instability. However, for everyday use in clinical practice, the diversity of minimally invasive hemodynamic monitoring requires knowledge of a number of different techniques, their operating concepts, their settings, and their respective clinical validity. In the first part of the present book, we present the hemodynamic monitoring parameters available to the clinician and their pathophysiological importance. For instance, blood pressure is the basic parameter, but measuring the arterial tone is sometime also necessary [7]. Additionally, measuring the intravascular pressure [8], the cardiac output, and their derived parameters is essential to determine and manage a balance between oxygen supply and consumption [9]. In this regard, we review techniques for cardiac output measurements based on pulmonary thermodilution [10], transpulmonary thermodilution [11, 12], echocardiography [13, 14], and Doppler techniques [15]. We discuss the techniques based on calibrated and non-calibrated pulse contour analysis [16] and their limitations. Finally, we discuss the dynamic indices of fluid responsiveness and their clinical applications and issues. Geneva, Switzerland Raphael Giraud Karim Bendjelid