Stem cells have now been described in a variety of tissues, even in those where the cells' turn over rate is slow, such as the brain and the resting mammary gland. There is also accumulating evidence that tumors are derived from and are maintained by a rare population of dysregulated stem cells. However, discrepancies in the markers used and reported have slowed down the functional characterization of these somatic stem cells. To circumvent this challenging issue, universal stem cell markers with properties common to all stem cell types must be discovered and exploited. In line with this idea, the measurement of aldehyde dehydrogenase isoform 1 (ALDH1) activity shows promising potential as a universal marker for the identification and isolation of stem cells from multiple sources. Herein, we review the available data reporting utilization of ALDH1 activity as a means to identify and isolate stem cells and cancer stem cells, with a special focus on the mammary gland and breast cancer.