Hematopoietic stem cell (HSC) subsets are defined by the capacity to which their offspring can contribute to the various mature blood-cell lineages. However, the proliferative potential of stem cells is highly dependent on the environment in which they reside, and it is only in retrospect that the characteristics of a stem cell can be identified. Partly because of these elusive properties of stem cells, various functional assays to measure their frequency have been developed, both in vivo and in vitro. Various chapters in this volume each describe one of these assays. All these assays have the ability to quantify stem cells with certain properties, but not others. Therefore, the determination of which stem-cell assay will be most appropriate to use is highly dependent on a particular experimental setting. This chapter describes the cobblestone-areaforming cell (CAFC) assay (1).