Proliferation in mouse oesophageal epithelial cells is confined to the basal layer of the epithelium. Within this population, it is possible to discriminate different sub-populations using a combination of cell kinetic studies and functional assays. In particular, it is possible to distinguish basal epithelial cells, which are post-mitotic and destined to leave the basal layer and differentiate compared with those cells that remain in the cycling pool. Within the cycling basal population, there appears to be a hierarchy with respect to the rate of cell turnover which may reflect a hierarchy of "stemness", although it has not been possible to demonstrate functional differences between these populations using current in vivo tissue reconstitution assays. The aim of this chapter is to describe the development of a quantitative in vivo tissue reconstitution assay to assess the potency of candidate stem cell populations within the mouse oesophageal epithelium.