This chapter reviews the estimated thrombotic risks associated with the oestrogen content of combined hormonal contraception. Thrombotic diseases discussed in this chapter are arterial thrombosis and venous thrombosis. The risk of arterial thrombosis, including stroke and myocardial infarction, is reported to be increased in users of combined hormonal contraception. Such events may be fatal, or lead to disabling sequelae. While oestrogen may play a role in arterial thrombosis, the effect is primarily related to an interaction with traditional, and to some extent modifiable, risk factors for arterial disease. Venous thrombosis mostly manifests in the deep veins of the leg, but may occur in other sites, such as the upper extremities, cerebral sinus, liver and portal veins or retinal veins. The risk of venous thromboembolism (VTE) is strongly associated with age, obesity and in users of oral contraceptives (OCs). Pregnancy is a far more profound thrombophilia risk.