Rapidly accumulating evidence points to the matrix metalloproteinases (MMPs) as major molecular mediators of arterial diseases. Findings from human pathological specimens, animals, and cell and molecular biology implicate matrix metalloproteinases in all stages of atherosclerosis including lesion initiation and progression and ultimately in plaque complication and triggering of thrombosis. The complex interactions within the proteolytic cascade allow multiple levels of control over these functions. This review weighs the evidence for the role of MMPs in arterial biology with particular reference to their activation in the atherosclerotic plaque.