The genesis of mesodermal tissues, such as bone, cartilage, muscle, marrow stoma, tendon, fat, dermis, and connective tissues, in either embryos or adult organisms is referred to as the mesengenic process. The process is involved in the continual rejuvenation of each mesenchymal tissue and provides a logic for understanding the rapid repair of tissue injuries. The isolation and identification of mesenchymal stem cells and their quantity and distribution allow a better understanding of the turnover and repair motifs used in adult organisms. Mesenchymal stem cells undergo proliferative and commitment events, and their progeny enter discrete lineages that result in the ultimate terminal differentiation of definitive phenotypes such as osteoblasts, chondrocytes, or myoblasts. The molecular and cellular control of the mesengenic process provides a logic for designing cell therapies for the reconstruction of mesenchymal tissues.