Extracorporeal and mechanical circulatory support devices are commonly used in the sickest patients with cardiac and respiratory disease, but the evidence base that guides their use is lacking. This thesis defines many contemporary practices used with these devices, as well as the common definitions and outcome measures used in the medical literature. The thesis also describes several different mechanisms of how these devices interact with the patient’s body. This work forms the basis of ongoing studies, including an internationally recognised core outcome set, and a binational registry.