Background: The vertebrate body axis extends sequentially from the posterior tip of the embryo, fueled by the gastrulation process at the primitive streak and its continuation within the tailbud. Anterior structures are generated early, and subsequent nascent tissues emerge from the posterior growth zone and continue to elongate the axis until its completion. The underlying processes have been shown to be disrupted in mouse mutants, some of which were described more than half a century ago. Results: Important progress in elucidating the cellular and genetic events involved in body axis elongation has recently been made on several fronts. Evidence for the residence of self-renewing progenitors, some of which are bipotential for neurectoderm and mesoderm, has been obtained by embryo-grafting techniques and by clonal analyses in the mouse embryo. Transcription factors of several families including homeodomain proteins have proven instrumental for regulating the axial progenitor niche in the growth zone. A complex genetic network linking these transcription factors and signaling molecules is being unraveled that underlies the phenomenon of tissue lengthening from the axial stem cells. The concomitant events of cell fate decision among descendants of these progenitors begin to be better understood at the levels of molecular genetics and cell behavior. Conclusions: The emerging picture indicates that the ontogenesis of the successive body regions is regulated according to different rules. In addition, parameters controlling vertebrate axial length during evolution have emerged from comparative experimental studies. It is on these issues that this review will focus, mainly addressing the study of axial extension in the mouse embryo with some comparison with studies in chick and zebrafish, aiming at unveiling the recent progress, and pointing at still unanswered questions for a thorough understanding of the process of embryonic axis elongation. Developmental Dynamics 243:88–98, 2014. © 2013 Wiley Periodicals, Inc.