Aims. This thesis examines the potential of Human Bone Marrow Stromal Cell (HBMSC) fractions to augment bone formation after impaction bone grafting. The project will examine the biological response of marrow derived cells on washed morsellised allograft, Beta - Tricalcium Phosphate (P-TCP) and poly(DL-lactic) acid (PLA) synthetic bone graft substitutes in an impaction grafting model. Study Design. Study I: In - vitro analysis of unselected HBMSC populations seeded onto natural (allograft) and synthetic (P-TCP and PLA) scaffolds after impaction. Study II: Parallel in - vitro and in - vivo analysis of unselected and immunoselected HBMSC populations seeded onto natural (allograft) and synthetic (PLA and P-TCP) scaffolds after impaction. Study III: Proof of concept clinical translation of methods from laboratory to operating theatre utilising autologous HBMSCs seeded onto washed allograft and impacted into two patients with proximal femoral defects.