The mammary gland is the only organ to undergo most of its development after birth and therefore particularly attractive for studying developmental processes. In the mouse, powerful tissue recombination techniques are available that can be elegantly combined with the use of different genetically engineered mouse models to study development and differentiation in vivo.In this chapter, we describe how epithelial intrinsic gene function can by discerned by grafting mammary epithelial cells of different genotypes to wild-type recipients. Either pieces of mammary epithelial tissue or dissociated mammary epithelial cells are isolated from donor mice and subsequently transplanted into recipients whose mammary fat pads were divested of their endogenous epithelium. This is followed by phenotypic characterization of the epithelial outgrowth either by fluorescence stereomicroscopy for the fluorescently marked grafts or carmine alum whole mount for the unmarked epithelia.