Extracorporeal circuits are increasingly used to support critically ill patients with severe cardiac and/or respiratory failure. They may be used as a bridge to recovery, transplantation, decision for further intervention, or, in a very few patients with cardiac failure, as destination therapy. Although echocardiography for extracorporeal support is highly specialist, certain key principles apply. First, extracorporeal support is not a treatment per se, but rather a supportive therapy whilst awaiting resolution of the underlying pathological process. Thus echocardiography has a vital role in excluding any potentially treatable underlying cause for cardiorespiratory failure. Second, echocardiography is required to determine the requirement for right and/or left ventricular support, the level of support required, and assessing the ability of the right and left ventricles to support the extracorporeal circuit. This demands that the practitioner understands the different types of circuit, and the load that each will place on the heart. Third, echocardiography is mandatory to exclude cardiovascular contraindications to initiation of support. Echocardiography subsequently has a vital role in its successful implementation, including confirming/guiding correct cannula placement, ensuring the goals of support are met, detecting complications, and assessing tolerance to assistance. Finally, in patients requiring extracorporeal cardiac support, various echocardiographic parameters have been proposed to be used in conjunction with clinical and haemodynamic assessment in order to attempt to predict those patients who can be successfully weaned.