**1. Introduction**

This chapter provides a summary of the current literature addressing the importance of vascular wall-resident multipotent stem cells within the process of vascular remodelling. First, the role of pericytes and smooth muscle cells (SMC) causing stabilization of angiogenic tumour vessels will be discussed at the molecular and cellular level. This stabilization phase is crucial for the survival of newly formed vessels, as immature vessels may rapidly become subject to regression and cell death when the angiogenic stimulus is removed. The second part of the chapter will focus on vascular wall-resident multipotent stem cells and evaluate the contribu‐ tion of circulating progenitor cells versus vessel-resident stem cells in the generation of pericytes and SMC within the neovascularization process. Here, the hypothesis will be proved that tissue-resident multipotent stem cells which putatively reside within the vascular adventitia, rather than circulating multipotent stem cells, are the major source for pericytes and SMC in the vascular stabilization processes. Finally, the regulation of differentiation of vascular wall-resident multipotent stem cells into SMC will be discussed.

Aspects of vascular stabilization, e.g., some decisive factors for the mobilization of vesselresident stem cells and differentiation into pericytes and SMC, may have the potential for clinically relevant applications in themselves. A better understanding of the molecular processes in these cells could lead to the identification of new therapeutic targets.
