Normal human and mouse bone marrow is composed of hematopoietic and nonhematopoietic cells. The latter have also been termed stromal cells, microenvironment cells, colony-forming-unit fibroblasts (CFU-F), and mesenchymal cells. These cells were originally thought to provide an appropriate matrix for hematopoietic cell development, but recent examination of these cell populations suggests a much broader spectrum of activity, including the generation of bone, cartilage, muscle, tendon, and fat. In the future, these mesenchymal cell populations could be used for the treatment of specific diseases and to enhance the engraftment of hematopoietic cells. This chapter describes methods for the human CFU-F assay, culture and expansion of mesenchymal cells, as well as their differentiation to adipocytes. In addition, this chapter describes the mouse CFU-F assay.