Embryonic stem (ES) cells derived from the inner cell mass of the blastocyst exhibit the remarkable capacity to differentiate into all cell types of the body. ES cells are capable of unlimited, undifferentiated proliferation in vitro, while still maintaining the capacity for development into a wide variety of somatic and extra-embryonic tissues. The integration of a vast array of environmental cues and signal transduction events orchestrates these processes. Recently it has been shown that adult cells can adopt a phenotype similar to ES cells by the introduction of a discrete subset of transcriptional regulators. Although more is yet to be learned, these induced pluripotent stem (iPS) cells show great promise as an alternative source for ES-like cells. Much research is focused on studying the factors that regulate ES/iPS cell differentiation with the hope that in the future these cells might be commonly used for therapeutic purposes. Part of this research includes the ability to identify each distinct stem cell lineage. Most often, the exquisite sensitivity of antibodies is used to assess the expression of markers specific for a given cell type. This illustration highlights a range of markers expressed as pluripotent stem cells develop along ectodermal, mesodermal, and endodermal lineages.