Publisher Summary This chapter provides an introduction to mesenchymal stem cells (MSCs). MSCs are widely defined as a plastic-adherent cell population that can be directed to differentiate in vitro into cells of osteogenic, chondrogenic, adipogenic, myogenic, and other lineages. MSCs proliferate and give rise to daughter cells that have the same pattern of gene expression and phenotype and, therefore, maintain the “stemness” of the original cells. Self-renewal and differentiation potential are two criteria that define MSCs as real stem cells. MSCs exhibit the potential to differentiate into the osteogenic, chondrogenic, adipogenic, tenogenic, myogenic, or stromal lineages. Application of MSCs requires their isolation and directing the differentiation of these cells into the appropriate lineage. Immunoisolation is a method to isolate noncultured MSCs based on cell surface markers. Immunodepletion is a “negative selection” approach in which the MSC population is enriched by washing out the cells labeled with antibodies, mostly directed against hematopoietic markers. Recently, more specific and pure populations are isolated utilizing a combination of immunoisolation and immunodepletion based on different surface markers. MSCs can either be systemically administered using intravenous (iv) injection or directly implanted in the bone defect site. The systemic approach assumes that MSCs have the capability of migrating across the endothelium and homing to injured tissues in a manner similar to the migration of leukocytes to sites of inflammation.