Various assays exist that measure the function of hematopoietic stemcells (HSCs). In this chapter, in vitro assays are described that measure the frequency of progenitors (colony-forming unit in culture; CFU-C), stem cells (long-term culture-initiating cell; LTC-IC), or both (cobblestone area-forming cell assay; CAFC). These assays measure the potential of a test cell population retrospectively, i.e., at the time its activity is evident when the stem cell itself is often not detectable anymore. Although the in vitro LTC-IC and CAFC assays have been shown to correlate with in vivo activity, in vivo transplantation assays, where it can be shown that cells possess the ability to indefinitely repopulate all blood lineages, are the ultimate proof for HSC activity. Nevertheless, these in vitro assays provide an excellent method to screen for stem cell activity of a putative stem cell population or for screening the effect of a certain treatment on HSCs.