Maintenance of the prostatic epithelial cell compartment is ensured by proliferation of adult epithelial progenitor or stem cells. These cells are characterized by an undifferentiated state, high proliferative capacity and long life span. Prostate progenitor/stem cells are localized in their stem cell-niche in the basal cell compartment in close contact to the basement membrane and the stromal cell compartment and are characterized by expression of the basal cytokeratins 5 and 14, high levels of integrins, CD44, the stem cell markers CD133 and ABCG2, and AR negativity. They give rise to secretory luminal (cytokeratins 8/18, CD57, AR, p27, PSA, PAP) and neuroendocrine cells (cytokeratins 8/18, CD57, CgA, NSE, NEPs), the two major cell types observed in the glandular epithelium. A growing body of experimental evidence has identified the amplifying progenitor/stem cell (CD44(+), alpha(2)beta(1)(hi), CD133(+)), as a putative origin of prostate cancer. Differentiation of this cell type can be affected by mutations in the intrinsic genetic program, by age-related changes in stromal-epithelial interactions or in the basement membrane/ECM composition. All these stochastic events occur during aging and can transform a normal prostate progenitor/stem cell into a cancer stem cell, a source of androgen-dependent and independent tumor cell clones. Thus, the heterogeneous and multifocal nature of prostatic cancer with a pleora of different tumor cell clones clearly reflects the differentiation capacity of the prostatic epithelial progenitor cells.