An ability to measure cardiac output in a continuous and non-invasive fashion is eagerly awaited in the field of intensive care practice. Modern technologies purport to be able to do this, but the design of studies that validate the manufacturers' claims is by no means straightforward. It is imperative that the scientific community describes and agrees on a set of principles that will enable us to design and then review and assess future validation studies, so that new technologies can be fairly assessed and compared with their competitors.