Hematopoiesis is the process by which stem cells divide and differentiate to produce the multiple types of mature cells found in blood. The process begins in early embryonic development and continues throughout adult life, primarily in the bone marrow. Various in vivo and in vitro assays have been developed to detect and assess stem cells and early multi-potential progenitors. While highly informative about primitive hematopoietic cells these assays are long and labour intensive. Alternatively, colony-forming cell (CFC) assays may be used to quantify more lineage-restricted progenitors in a simple in vitro assay. When cultured in a semi-solid medium containing the appropriate cytokines, CFCs are able to divide and differentiate into a colony of more mature cells that can be detected by light microscopy. This allows for the quantification of erythroid, myeloid, lymphoid, megakaryocytic, and multi-potential cell lineages from various cell sources. This chapter outlines the materials and methods used for the culture and assessment of CFC from humans, mice, and other species.