The microenvironment of breast cancer hosts a dynamic cross talk between diverse players of the immune system. While cytotoxic immune cells are equipped to control tumor growth and metastasis, tumor-corrupted immunosuppressive immune cells strive to impair effective immunity and promote tumor progression. Of these, regulatory T cells (Tregs), the gatekeepers of immune homeostasis, emerge as multifaceted players involved in breast cancer. Intriguingly, clinical observations suggest that blood and intratumoral Tregs can have strong prognostic value, dictated by breast cancer subtype. Accordingly, emerging preclinical evidence shows that Tregs occupy a central role in breast cancer initiation and progression and provide critical support to metastasis formation. Here, Tregs are not only important for immune escape but also promote tumor progression independent of their immune regulatory capacity. Combining insights into Treg biology with advances made across the rapidly growing field of immuno-oncology is expected to set the stage for the design of more effective immunotherapy strategies.