Abstract: Therapies targeting endocrine receptors and human epidermal growth factor receptor 2 have become important treatment modes for patients with hormone receptor breast cancer. Despite the availability of these options, however, development of primary or secondary drug resistance and subsequent disease progression in patients with advanced disease continue to occur. Mammalian target of rapamycin (mTOR), a key regulator of cell growth and proliferation, has been implicated in the induction of cellular processes leading to the uncontrolled growth of cancer cells. Recent studies have suggested that overactivation of the mTOR pathway may be involved in the development of endocrine resistance. Interrupting this signaling cascade may alleviate such resistance and help restore drug sensitivity. A number of agents targeting the mTOR pathway have shown potent anti-tumorigenic effects in vitro, and several agents show great potential for treating breast cancer patients. Many clinical studies have shown that combining endocrine therapy with mTOR inhibitors could significantly increase the survival rate of breast cancer patients. In this study, we focus on recent research progress on mTOR and its inhibitors in endocrine therapy resistance in breast cancer.