Members of the insulin-like growth factor (IGF) family are mitogenic growth factors which have been shown to play critical roles in both normal growth and development, and tumour biology. The IGF system is complex and the biological effects of the IGFs are determined by diverse interactions between many molecules, including interactions with the extracellular matrix (ECM). Recent observations have demonstrated that IGFs can associate with the ECM protein vitronectin (VN) and this interaction can modulate IGF-stimulated biological functions. It has been demonstrated previously that IGF-II can bind directly to VN, while IGF-I associates with VN indirectly via the involvement of IGF-binding proteins (IGFBPs) -2, -3, -4 and -5. As the IGF system plays important roles in both normal breast development and in the transformation and progression of breast cancer, this study aimed to describe the effects of substrate-bound IGF-I:IGFBP:VN complexes on breast cell functions and to dissect the mechanisms underlying these responses. The studies reported in this thesis demonstrate that substrate-bound IGF-I:IGFBP:VN complexes, containing IGFBP-3 and IGFBP-5, are potent stimulators of proliferation and migration in the "normal", non-tumourigenic MCF-10A breast epithelial and MCF-7 breast carcinoma cell lines. Interestingly, substrate-bound IGF-I:IGFBP:VN complexes were less effective in increasing the migration of the metastatic MDA-MB-231 breast cancer cell line. This, however, is due to these cells expressing the αvβ3 integrin which can support a highly migratory phenotype independent of IGF-I-stimulation. Taken together this suggests a particularly important role for these complexes in stimulating a highly migratory phenotype in pre-invasive or poorly metastatic breast cells. Studies using IGF-I analogues were also undertaken to establish if there was a requirement for ternary complex formation and the type-1-IGF receptor (IGF-1R) in the enhanced migration responses observed. These studies determined IGF-I:IGFBP:VN-stimulated migration to be dependent upon both heterotrimeric IGF-I:IGFBP:VN complex formation and activation of the IGF-1R. Furthermore, the enhanced cellular migration was abolished upon incubation of MCF-7 and MCF-10A cells with function blocking antibodies directed at VN-binding integrins and the IGF-IR. In addition, analysis of the signal transduction pathways underlying the enhanced cell migration revealed that the complexes stimulate a transient activation of the ERK/MAPK signaling pathway, while simultaneously producing a sustained activation of the PI3-K/AKT pathway. Optimal intracellular signaling required activation of both the IGF-1R and VN-binding integrins, as antibody mediated inhibition of either receptor led to substantial decreases in both ERK/MAPK and PI3-K/AKT pathway activation. Furthermore, experiments using pharmacological inhibitors of these pathways determined a pivotal role for PI3-K/AKT activation in substrate-bound IGF-I:IGFBP:VN-stimulated cell migration. In order to confirm an important role for the PI3-K/AKT pathway in these responses, wild-type and activated-AKT was transiently overexpressed in MCF-10A cells. Overexpression of both wild-type and activated-AKT further enhanced cellular migration in response to substrate-bound IGF-I:IGFBP:VN complexes. However, these responses still required co-activation of the IGF-1R and VN-binding integrins. In an attempt to obtain a global view of the possible molecular mechanisms underpinning IGF-I:IGFBP:VN-stimulated cell migration, oligonucleotide microarrays were used to screen for candidate genes important for the observed migratory responses. The microarray studies identified 165 genes which were differentially expressed in cells migrating in response to substrate-bound IGF-I:IGFBP:VN complexes. Gene ontology and functional analysis revealed many of these genes to be significantly associated with biological functions relevant to cancer transformation and progression, including cell growth and proliferation, cell death and cellular movement. In regard to cell migration, a number of the genes identified have previously reported roles in cellular movement, migration and metastasis, which may provide future targets to augment IGF-I:IGFBP:VN-stimulated cell migration. Taken together, the studies reported throughout this thesis have provided the first mechanistic insights into the action of IGF-I:IGFBP:VN complexes and add further evidence to support the involvement of VN-binding integrins and their co-operativity with the IGF-IR in the promotion of tumour cell migration. Importantly, identifying the molecular mechanisms by which IGF:VN complexes enhance breast cell function will lead to not only a better understanding of this critical interaction, but also aid in developing diagnostic tests and therapeutics directed at treating breast cancer.