CSCs resemble cells that have undergone epithelial–mesenchymal transition (EMT) with regard to their invasiveness and motility. EMT, which is associated with a key step in tumor metastasis via induction of a highly invasive phenotype, has been intensely studied. And CSCs have been found to transiently acquire stem cell-like properties as a consequence of EMT. CSCs also are inherently chemoresistant and radioresistant. Recently, side population (SP) cells have been shown to express increased levels of MDR1 (multidrugresistance protein 1) and ABCG2 (ATP-binding cassette, sub-family G, member 2) transporters, which can extrude certain chemotherapeutic agents and have been implicated in drug resistance. Importantly, SP cells appear to be enriched in stem cells. Although this issue remains controversial, identifying and characterizing human ovarian CSCs would provide a critical step toward the identification of drugs with therapeutic efficacy against CSCs and the development of novel therapeutic strategies for the long-term control of cancer.