**1. Introduction**

Mesenchymal stem cells (MSCs) are adult, multipotent stem cells endowed of self-renewal, a process of continuous divisions essential to maintain the stem cell pool. Meanwhile, MSCs can be activated under the action of growth factors, chemokines and cytokines which are normally released during the physiological tissue renewal or in pathological conditions in the presence of tissue damage. Specific signals stimulate the MSC migration at the damaged site and their differentiation into specialized cell types belonging to the mesodermal lineage. The homing and the differentiation potential allow MSCs to be actively involved in the tissue homeostasis as well as in the repair process. MSCs have been firstly identified as a nonhematopoietic, adherent and spindle-shaped cell subset of the human bone marrow stroma [1]. From their first isolation in 1970, MSCs have been extensively characterized, gaining the increasing interest of the scientific community, and lots of studies issued the biology and the inner properties of these promising cells. One of the most intriguing and studied functions of MSCs is the immunomodulation, that is, the ability to repress inflammation; however, little is known about the reversal of

this property that has been observed in some disease models. The present chapter will review the differentiation and immunomodulatory capabilities of MSCs, will discuss the contradictory face of MSCs and will focus on the vascular wall setting.
