SummaryDepression is an illness that kills. The links between depression and medical illness are well established and bi-directional, but evidence is mounting that depression increases mortality as well as morbidity in adults, particularly older adults. We examine the evidence that the increase in mortality in depression applies to all-cause mortality as well as cardiac mortality, and describe plausible physiological theories for the association. We conclude that excess mortality arising from depression is a major public health problem that is largely unrecognised and needs to be addressed by a range of clinicians.