Putative “stem” cells (SCs) are increasingly envisioned and postulated in discussions and experimental work dealing with the origin and differentiation of postnatal mesodermal tissues. In the age of Dolly (a lamb born from the nucleus of a somatic cell), basic traditional concepts of cell lineage, commitment, and differentiation are obviously challenged. The current appreciation of the “diversity” of stem cells (whereby no single defining feature of a stem cell applies to all stem cells [40]) discourages general assumptions on stem cell nature and function. Stem cells can only be profitably investigated with respect to the function they subserve in the tissue where they belong, that is, in specific rather than general terms. Progenitors of solidphase mesodermal tissues may indeed be as different from hematopoietic stem cells (HSCs), the best-known stem cells (see Orkin [42] for a recent review), as are the developmental and physiological dynamics of the respective tissues (hematopoietic vs muscle and connective tissues). A demonstration of the capacity of mesodermal tissue progenitors to fully reconstitute a depleted animal for its lifetime (which is typical of HSCs and other well-characterized SCs) is still missing; for this reason we will refer from now on to these cells as progenitors and not as stem cells. Even with the limits of present uncertainty, the presumed role of these progenitors in tissue development and repair, as well as the option of manipulating them ex vivo, discloses exciting glimpses on their potential use for somatic cell and gene therapy [14, 49]. Still, significant controversy exists over basic issues concerning the origin, identity, mobility, and mutual relationships of progenitors of different mesodermal tissues. Addressing these issues may lead to a better understanding of their biology and thus benefit technological developments as well. Here, we propose a new model for the origin, nature, and identity of postnatal mesodermal progenitors, which attempts to reconcile a vast and disperse series of observations in different developmental stages of different mesodermal systems.