Depression is disproportionately prevalent among cardiac patients, with estimates of Major depressive disorder (MDD) of about 15% in patients following acute myocardial infarction (AMI) or coronary artery bypass graft (CABG), and an additional 20% with either minor depression or elevated levels of depressive symptoms as measured by questionnaires such as the Beck Depression Inventory (BDI). This chapter describes the evidence that depression is a risk factor in patients with established Coronary heart disease (CHD) and suggests potential mechanisms underlying the relationship between depression and adverse outcomes. It focuses on evidence that depression in patients with existing CHD poses a risk for increased morbidity and mortality. A number of biobehavioural mechanisms have been hypothesised to underlie the relationship between depression and CHD. Cross-sectional studies have demonstrated increased levels of Creactive protein (CRP) and other inflammatory markers in patients with depression and with other CHD risk factors, such as the metabolic syndrome.