Recent technical progress in the field of cancer stem/progenitor cell research revealed that these malignant cells may provide critical roles for primary tumor growth, metastases at distant tissues and organs, treatment resistance, and disease relapse. The precise molecular oncogenic events that frequently occur in cancer stem/progenitor cells and their early progenies during the early and late stages of cancer progression as well as their contribution to the treatment resistance and disease recurrence remain poorly defined. This lack of information on the deregulated gene products that may be involved in the malignant transformation of tissue-resident adult stem/progenitor cells into highly tumorigenic and/or migrating cancer stem/progenitor cells emphasizes the urgent need to perform future investigations. Toward this direction, we describe in this book chapter the characterization of nonmalignant and malignant prostatic stem/progenitor cells from well-established cell lines by Hoechst side population method. This novel approach should help to establish novel in vitro and in vivo models of human cancer stem/progenitor cell mimicking more closely the genetic and phenotypic changes occurring during the different stages of prostate carcinogenesis and disease progression in clinical settings. Of therapeutic interest, the identification of new biomarkers and molecular targets specific to these prostatic cancer-initiating cells should also help to develop more effective diagnostic and prognostic tests and chemopreventive and therapeutic treatments for the patients diagnosed at early and late stages of disease progression.