INTRODUCTIONThe generation of functionally competent T-cells from their progenitors involves a series of developmental events including proliferation, differentiation, and survival. T-cell development is a non-cell-autonomous event, and requires interactions with thymic stromal cells. Fetal thymus organ cultures provide an in vitro system in which isolated embryonic thymus lobes can be maintained in culture, allowing the study of T-cell development as well as thymic stromal cell function. This system remains the only in vitro system that supports a complete program of T-cell development, including positive and negative selection of the developing T-cell receptor repertoire. Modifications of the basic fetal thymus organ culture system, such as hanging drop cultures and reaggregate thymus organ cultures, provide useful systems to analyze thymus colonization and thymic stromal cell function, respectively.