Physical examination alone can provide a wealth of information about the presence, nature and severity of cardiorespiratory disease; however, findings are subjective and may on occasion be misleading. Arterial blood gas analysis and pulse oximetry are objective methods used to identify and quantify hypoxaemia and to assess progression over time. Arterial blood gas analysis also provides objective information about ventilation. The chapter covers the following: Arterial blood gas analysis; and Pulse oximetry.