Although mortality rates from coronary heart disease in the western countries have declined in the last few decades, morbidity caused by this disease is increasing and a substantial number of patients still suffer acute coronary syndrome (ACS) and sudden cardiac death. Acute coronary syndrome occurs as a result of myocardial ischaemia and its manifestations include acute myocardial infarction and unstable angina. Culprit plaque morphology in these patients varies from thrombosis with or without coronary occlusion to sudden narrowing of the lumen from intraplaque haemorrhage. The coronary artery plaque morphologies primarily responsible for thrombosis are plaque rupture, and plaque erosion, with plaque rupture being the most common cause of acute myocardial infarction, especially in men. Autopsy data demonstrate that women <50 years of age more frequently have erosion, whereas in older women, the frequency of rupture increases with each decade. Ruptured plaques are associated with positive (expansive) remodelling and characterized by a large necrotic core and a thin fibrous cap that is disrupted and infiltrated by foamy macrophages. Plaque erosion lesions are often negatively remodelled with the plaque itself being rich in smooth muscle cells and proteoglycans with minimal to absence of inflammation. Plaque haemorrhage may expand the plaque rapidly, leading to the development of unstable angina. Plaque haemorrhage may occur from plaque rupture (fissure) or from neovascularization (angiogenesis). Atherosclerosis is now recognized as an inflammatory disease with macrophages and T-lymphocytes playing a dominant role. Recently at least two subtypes of macrophages have been identified. M1 is a pro-inflammatory macrophage while M2 seems to play a role in dampening inflammation and promoting tissue repair. A third type of macrophage, termed by us as haemoglobin associated macrophage or M(Hb) which is observed at site of haemorrhage also can be demonstrated in human atherosclerosis. In order to further our understanding of the specific biological events which trigger plaque instability and as well as to monitor the effects of novel anti-atherosclerotic therapies newer imaging modalities in vivo are needed.