Endothelial progenitor cells (EPCs) reside in the bone marrow and are mobilized into the circulation by specific stimuli such as certain drugs, ischemia, and exercise training. Once in the circulation EPCs are thought to participate in the maintenance of the endothelial cell layer. Recently it was clearly demonstrated that the amount and function of EPCs is significantly impaired in different cardiovascular diseases. Furthermore, the level of circulating EPCs predicts the occurrence of cardiovascular events and death from cardiovascular causes and may help to identify patients at increased cardiovascular risk. After demonstrating the beneficial effect of applied EPCs in several animal experiments, these cells were also used to treat humans with different cardiovascular diseases. This review will focus on the characterization and liberation of EPCs from the bone marrow, as well as on the most important clinical cardiovascular diseases for which EPCs were used therapeutically.