Breast cancer is a heterogeneous disease, and can be classified according to histological subtypes based on cellular morphology. Invasive ductal carcinoma (IDC) and invasive lobular carcinoma (ILC) are the most common histological subtypes, accounting for approximately 80% and 12% of cases respectively. ILC exhibits a number of distinct clinico-pathological features in comparison with IDC, and is understudied as a breast cancer subtype. ILC tumours are typically oestrogen receptor positive, HER2 negative, and frequently demonstrate early loss of Ecadherin expression, which is a hallmark of the lobular phenotype. ILC is presently treated in a similar manner to IDC, with treatment generally directed against hormone receptors. Upon acquisition of hormone resistance, limited secondary options are available; patients are rarely candidates for agents targeting HER2, and are recognised to be poorly responsive to chemotherapeutics. We therefore need to advance our understanding of lobular tumour biology, in order to identify suitable biomarkers that will guide the development of targeted therapies for ILC patients. As protein expression levels determine cellular phenotype, a protein-based approach has the potential to provide biologically relevant insight into the mechanisms driving ILC. A range of protein analysis platforms, including reverse phase protein array, label-free mass spectrometry and immunohistochemistry, were therefore used to elucidate biological mechanisms active in the ILC subtype. Such experiments led to the identification of activated PI3K-Akt signalling in mouse and human ILC, suggesting that inhibition of this pathway may be an effective treatment strategy in lobular breast cancer. Preliminary evidence of differences in cytoskeletal and extracellular matrix (ECM) proteins was also acquired, providing an interesting basis for future research. A further major strand of this project was the development of in vitro and in vivo tools, to facilitate further interrogation of lobular biology. This included determination of a representative mouse model of ILC, and generation of primary cancer cells and cancer-associated fibroblasts (CAFs) from patient-derived material. Analysis of CAFS showed differential expression of ECM-associated genes, consistent with proteomic analyses. In addition, a tissue micro-array (TMA) comprising primary ILC and IDC tumours, with associated clinical data, was developed. Immunohistochemical staining of the TMA identified a potential role for IGF-1 pathway signalling in ILC, with increased expression of IGF-1 ligand associating with increased tumour size and metastasis in ILC patients. Taken together, the generation and validation of a range of useful tools in the course of this work has provided useful insight into the unique biology of ILC.