Most ICU patients have a significant number of risk factors for VTE. The high incidence of DVT in the ICU population and the recognition of a high incidence of PE at autopsy confirm this. We have alluded to the difficulty of clinical diagnosis of VTE and the need for diagnostic investigations. We have reviewed currently available diagnostic investigations with regard to their sensitivity and specificity and their practicability in ICU patients, and have formulated recommended diagnostic algorithms (Figs. 4 and 5). The most important factor in the management of VTE is prevention. In the ICU, all patients are at high risk for VTE, and therefore, at a minimum should receive subcutaneous prophylactic heparin unless it is contraindicated. Alternative methods of prophylaxis are available, and should be considered for patients who have contraindications to heparin.