Stem cells are extremely rare cells with two major characteristics: the capacity for self-renewal and the ability to differentiate into mature daughter cells. New scientific evidence suggests that human tumors contain a subset of cells that, unlike the majority of cells of which the cancer is composed, can propogate tumors within the host and generate new tumors when transplanted into mice. These so-called "cancer stem cells" may represent a new target for cancer therapy in the future. The idea is that instead of treating all cancer cells non-specifically, targeting the specific cancer cells with the ability to propogate tumors may lead to more effective control of disease. Because stem cells represent such a rare minority population within normal tissue or tumor, they have been extremely difficult to isolate and characterize. Recently, a new technique for cultivating stem cells has been described in which fresh tissue is dissociated into single-cell suspensions. Under certain culture conditions, a subpopulation of cells from dissociated tissue can grow as spheres, or balls of cells, that contain a mixture of stem cells and early progenitor cells. These spheres can be further dissociated and passaged, leading to the generation of new spheres. Alternatively, under the influence of certain growth or stimulating environments, cells from spheres can differentiate into the variety of cell types contained within the organ or tumor. By generating spheres, it will be possible to characterize the cell populations that are responsible for tissue regeneration and/or cancer formation. This, in turn, suggests major implications in the future treatment of cancer, in that new targets may be identified for neutralization and elimination of disease.