The identification of endothelial progenitor cells (EPCs) in the adult human has forced us to reconsider how new blood vessels grow in physiological and pathological conditions in the adult human. An important question in angiogenesis research is to what extent these progenitors functionally contribute to revascularization of ischemic tissues and and to what extent they can be used for therapeutic angiogenic cell transplantation. Even more challenging is the concept that hematopoietic and other bone-marrow-derived stem cells might be recruited in the context of ischemia to induce neovessel formation. This review discusses some of the recent insights and outstanding questions on EPCs, both from a biological and therapeutic perspective.