Prevention of venous thromboembolism is of major importance because deep vein thrombosis is an economic burden. To prevent pulmonary embolism, whether fatal or not, and the postphlebitic syndrome, virtually all patients' level of risk should be assessed in order to provide adequate prophylactic measures against venous thromboembolism. Non-pharmacological, pharmacological or combined modalities can reduce the frequency of venous thrombosis. Evidence-based guidelines are available for most situations in surgical patients. However, in medical patients there are fewer data and there are wide variations of opinion. Systematic reviews should be performed and updated to obtain practice guidelines. Cost and effectiveness as well as patients' preferences should be taken into account. Randomized control trials are ongoing: low-molecular-weight heparins are being evaluated in general medical patients; other forms of prophylaxis or combined methods are also being investigated.