Metastasis is the primary cause of deaths in cancer patients. Tumour dissemination can occur through the bloodstream or lymphatic system. The presence of tumour cells in the draining lymph node (lymph node involvement) represents the most important prognostic factor for breast cancer patients. Despite its clinical implications, the mechanisms governing lymphatic metastasis remain poorly understood. This thesis aims to identify novel determinants of lymphatic dissemination in breast cancer. It focuses on the influences of the tumour immune microenvironment and the cancer cell-autonomous metastatic drivers associated with lymph node involvement. Using transplantable syngeneic mammary carcinoma 4T1 model, a differential preference for using the lymphatic system as a dissemination route was observed in immunocompetent mice when compared to immunodeficient mice. Characterisation of the differences in the tumour microenvironment revealed a higher infiltration of neutrophils in the tumours with lymph node metastasis. Depletion of neutrophils reduced lymphatic dissemination in 4T1 cells. In vitro, tumour-associated neutrophils were seen to promote tumour migration. Finally, single-cell RNA-seq analysis of lymph nodes suggests that neutrophils could have an immunosuppressive role in lymph nodes. Tumour cell-intrinsic gene signatures driving lymph node organotropism were investigated in subclonal lines derived from the 4T1 model with enhanced ability to seed lymph node metastasis. Transcriptomic profiling of these lines highlighted the role of metabolic adaptation and immunosuppression as central determinants of lymphatic metastasis. Finally, other syngeneic breast cancer models were used to assess the validity of the above-mentioned findings across breast cancer subtypes. Molecular characterisation of these indicated that immunosuppression, lipid metabolism and cell plasticity are common denominators associated with lymphatic metastasis among breast cancer models. This thesis advances our understanding of the involvement of neutrophils and tumour cell-intrinsic mechanisms that contribute to lymphatic metastasis in breast cancer. This work suggests potential gene and pathway targets that could be exploited therapeutically for the management of lymph node involvement.