Endothelial progenitor cell-based therapy offers great potential to reduce morbidity and mortality in patients with vascular diseases. In animal models of ischemia, circulating endothelial progenitor cells (EPCs) have been shown to home to sites of active angiogenesis and differentiate into endothelial cells in response to tissue ischemia, vascular trauma, or tumor growth. These studies indicate a thorough understanding of EPCs function and role in angiogenesis as a potential therapeutic target for vascular diseases. In this chapter, we describe the procedures for: (1) isolation of EPCs from circulating blood and bone marrow; (2) culturing these cells; (3) characterization of EPCs by FCAS, DiI-ac-LDL up-taking, and FITC-UEA-1 binding assays; and (4) determination of EPCs’ function by examining proliferation, migration, and tube formation in vitro.