Hemodynamic instability is one of the leading causes of admission to the ICU. Hemodynamic instability requires physiological and mechanical support to ensure there is adequate cardiac input and output, or blood pressure (BP). The goal of the critical care provider is to quickly identify the type of hemodynamic collapse present and to treat the condition appropriately to improve survival. This chapter provides a list of definitions and abbreviations commonly use in reference to hemodynamic monitoring equipment. Many providers are now searching for new technologies to aid the diagnosis of cardiovascular collapse. The chapter outlines many devices that will assist the provider in identifying specific parameters related to hemodynamic instability. Noninvasive monitoring may be limited by patient cooperation or body habitus. Invasive monitoring may be limited by poor circulation or misplacement of lines. It is essential that the healthcare provider provide careful interpretation in each individual case.