Cardiovascular disease claims more lives than any other disease in westernised countries, affecting millions. Pin-pointing CVS dysfunction is often difficult because the clinical signs, or the availability and interpretation of physiological measurements are unreliable. Often patient specific information is incomplete and/or confusing as it comes from a diverse range of sources such as invasive and non-invasive pressure measurements, flow rates and ECG signals. Health professionals therefore rely on intuition and experience to make a ‘clinical’ diagnosis and treatment decisions. Sometimes this approach results in multiple therapies being applied until a suitable treatment is found. Poor outcomes result from failure to quickly and correctly diagnose and treat the underlying condition. This monograph introduces the concept of using full circulatory and cardiovascular models to aggregate the large number of diverse signals facing clinicians into a clear physiological picture of haemodynamic status. A brief review of the field, still in its infancy, of such models is presented focusing primarily on the basic approaches taken in the literature. Finally, one of the more advanced and best validated models is presented including initial animal validation study results. The overall approach is shown to have significant potential to provide clear, measured insight to replace often misled intuition in the monitoring, diagnosis and treatment of circulatory dysfunction in critical care. In the future models and modern sensors will increasingly ‘invade’ the critical care environment and will provide the opportunity for better, more consistent care at the bedside, and in real time.