Thrombosis is the formation of a blood clot inside a blood vessel, obstructing the flow of blood through the circulatory system. When a blood vessel is injured, the body uses platelets and fibrin to form a blood clot, because the first step in repairing it (hemostasis) is to prevent loss of blood. If that mechanism causes too much clotting, and the clot breaks free, an embolus is formed. Thromboembolism is both thrombosis and its main complication, which is embolisation. This book presents and discusses recent research on the causes, treatment and prevention of thrombosis such as thromboembolic complications in childhood nephropathies, renal vein and artery thrombosis in children, management of portal vein thrombosis and others.