To better target the onset and progression of cancer, we need to understand cancer cells as well as the tumour microenvironment (TME). Cancer-associated fibroblasts (CAFs) are the main components of the TME and regulate carcinogenesis in different ways and at different stages of neoplasia. For instance, CAFs can modify the extracellular matrix (ECM) and exert immunomodulatory effects, either immunogenic or immunosuppressive. Breast cancer is the most frequent cancer found amongst Australian women and includes different subtypes based on molecular markers. CAF subsets found in breast cancer patients with different molecular profiles have been recently described, but functional characterisation of the types of fibroblasts that exist in the mammary gland during development is still lacking. Interestingly, women with high mammographic density (high collagen content in their breast ECM) have an increased risk of breast cancer. This suggests that breast fibroblast subtypes in healthy tissue can impact on breast cancer development. In this study, we aim to uncover the molecular pathways that define mammary fibroblasts during mouse mammary gland development in order to understand how they contribute to the tumorigenic process.