This chapter discusses the basic methods and principles of monitoring for proper management of the critically ill patient in the emergency setting. Pulse oximetry provides continuous measurement of a patient's oxygenation status in the case of respiratory monitoring. Capnography measures the partial pressure or concentration of expired carbon dioxide (CO2), the end-tidal carbon dioxide (EtCO2). Ultrasonography of the inferior vena cava (IVC) can be useful in determining fluid responsiveness during non-invasive hemodynamic monitoring. Invasive hemodynamic monitoring provides data via catheters inserted in central veins or arteries. Central venous pressure is obtained by placing a central venous catheter (CVC) in the internal jugular or subclavian vein. The CVP should be interpreted with caution in critically ill patients that have known heart disease or structural cardiac anomalies. Pulse pressure variation (PPV) and systolic pressure variation (SPV) can be used to determine fluid responsiveness in a mechanically ventilated patient.