There is great controversy over the origin and definition of murine endothelial progenitor cells (EPCs). EPCs are reportedly important for the repair and remodeling of the vasculature and are implicated in tumor angiogenesis. Many conflicting reports exist as to whether these EPCs arise from the bone marrow hematopoietic compartment or whether they are non-hematopoietic in origin, and these differences could be attributed to the wide variance in assays used to identify the cells and time points at which the data are collected. Recently, circulating murine EPCs have been characterized as CD45(-)CD13(+)CD117(+)FLK-1(+) expressing cells via flow cytometry and this phenotype varies from prior descriptions. This review will focus on the changing phenotypic definition of murine circulating EPCs and the evidence that has been published in support of the lineage of origin of circulating EPCs and the role EPCs play in tumor angiogenesis in the adult mouse.