Vascular remodelling occurs during many forms of cardiovascular disease (CVD). This vascular remodelling can be either symmetrical as observed in arteriosclerotic conditions (e.g. natural vascular aging, transplant arteriosclerosis and in-stent restenosis (ISR)), or asymmetrical as observed in atherosclerosis. An integral part of the pathology of symmetrical vascular remodelling is the formation of a “neointimal layer”, also known as “intimal medial thickening” (IMT), whereby cells (widely considered to be vascular smooth muscle cells (vSMC)) accumulate within the vessel and obstruct circulatory blood flow. Currently, there are two predominant and contrasting fields of thought as to the origin of these neointimal cells: 1. resident vSMCs and 2. resident vascular stem cells. While this thesis discusses both theories in detail; it focuses on the role of a stem cell antigen 1 positive (Sca1+) resident vascular stem cell, and shows evidence of its contribution to vascular remodelling both in vitro and in vivo. Previously, a Sca1+ adventitial cell has been shown to co-localise at the adventitial medial boundary with the hedgehog (Hh) signalling activation morphogen Shh and its receptor Patched 1. Moreover, Patched 1+ cells are present in the medial and intimal layers following vascular injury and remodelling. Therefore, the aim of this thesis was to investigate the role of Sca1+ adventitial progenitor cells (APC) during vascular remodelling, with a focus on exploring the role of Hh- responsive Sca1+ APC contribution to IMT. Characterisation of two widely used model cell lines (Sca1+ ESC and Sca1+ C3H) and APC highlighted variable expression profiles. Additionally, in vitro exposure of Sca1+ APC and Sca1+ C3H to Shh demonstrated contrasting maintenance of “stemness” responses (with Shh inducing significant maintenance of “stemness” in Sca1+ C3H but not in Sca1+ APC). Importantly, in vitro exposure of Sca1+ APC to Shh activated Hh signalling, and induced cellular proliferation and SMC differentiation, a process that was attenuated using the Hh specific inhibitor cyclopamine. Crucially, in vivo analysis demonstrated that Sca1+ APC proliferate/expand following injury-induced vascular remodelling, and Sca1+ Hh responsive cells were found in the neointima of remodelling vessels. In vivo analysis also demonstrated that the recapitulation of Hh signalling during vascular remodelling is essential to pathological IMT (as confirmed by significant attenuation of vascular remodelling following treatment with the cyclopamine). To conclude, Hh responsive Sca1+ resident vascular stem cells are key to the progression of pathological vascular remodelling and ultimately cardiovascular disease.