Telomerase activity is a feature of stem/progenitor cells and protects against senescence by maintaining telomere length during repeated cycles of cell division. Telomerase reverse transcriptase is the catalytic subunit of the telomerase complex and its expression correlates with telomerase activity.  Reporter constructs for mouse telomerase reverse transcriptase (mTert) have recently been demonstrated to mark stem and progenitor cells in the intestine and bone marrow. We have used mTert reporter mice to determine whether telomerase activity can also be used to identify adult stem and progenitor cells in the endometrium, the highly regenerative lining of the uterus. Green fluorescent protein expression driven by the mTert promoter marks an infrequent population of endometrial stromal cells during development and adulthood, and an even rarer population of endometrial epithelial cells in prepubertal and adult cycling mice. To investigate the differentiation potential of mTert expressing cells, Cre-recombinase dependent LacZ expression was used to permanently mark this population and its progeny. This lineage tracing approach demonstrates that mTert expressing cells contribute to glandular and luminal epithelia during development and the normal mouse estrous cycle.  To our knowledge, this is the first demonstration of the utility of mTert expression as a marker of stem/progenitor cells in the endometrium. The use of mTert reporter mice will provide insight into the role of progenitor cells in the normal development and cycling of the endometrium, and during inappropriate growth in conditions such as endometriosis.