Thrombosis can occur in both arteries and veins, but the mechanisms leading to it and the structure of the thrombus differ greatly. The main constituents of a thrombus blocking the veins are fibrin and red blood cells, whereas in the arteries, thrombi are made up of platelets and fibrin. In this chapter, epidemiology and mechanisms leading to a thrombus in both veins and arteries are discussed. Most recently, proteins (factors) favoring these events have been discovered, and effective treatments and prevention measures have been proposed. Thrombi, however, are not only the consequence of a mere activation of the coagulation mechanism, but lifestyle plays an important role, mainly within the frame of the ‘Metabolic Syndrome’* characterized by overweight, high blood pressure, hyperlipidemia, and reduced tolerance to sugars (prediabetes).