Treatment of women with oestrogen receptor-positive (ER-positive) early stage invasive breast cancer with adjuvant endocrine therapy for 5 years reduces recurrence rates in ER-positive breast cancer by about half and breast cancer mortality by about a third. Tamoxifen, a selective oestrogen receptor modulator is effective in premenopausal or postmenopausal women and can therefore be used regardless of the menopausal status of the patient. Aromatase inhibitors reduce the non-ovarian production of oestrogen and can be used in postmenopausal women to greatly reduce systemic oestrogen levels and thus to avoid stimulation of ER-positive breast cancer. Unlike most cancers, the risk of relapse for ER-positive invasive breast cancer remains significant even after completing 5 years of endocrine therapy. The aim of this review is to identify the optimal duration of endocrine therapy to minimise the risk of disease recurrence in women with ER-positive breast cancer. Adjuvant endocrine therapy for oestrogen positive (ER-positive) early breast cancer is well established. For premenopausal women, tamoxifen is the standard drug although the aromatase inhibitors can be given to premenopausal women if the ovaries are suppressed using gonadotropin-releasing hormone (GnRH) analogues or ablated by surgery or radiation. Theoretically, the absence of circulating oestrogen with ovarian function suppression/ablation (OFS) in addition to tamoxifen or switching to aromatase inhibitors (which are more efficacious in postmenopausal women) should improve long term outcomes including local and distant relapse from breast cancer. However, OFS has additional side effects for young women including menopausal symptoms with the potential for additional adverse effects on bone and cardiovascular health. International expert opinion (Burstein, 2016) suggests premenopausal women who receive chemotherapy or are considered high risk are offered OFS while the European Society for Medical Oncology (ESMO) Clinical Practice guidelines (Senkus, 2015) suggest a discussion with individual women based on risk and the potential side effect profile This review aims to determine the effectiveness of OFS in addition to endocrine therapy in premenopausal women. Ductal carcinoma in situ (DCIS) is non-invasive and considered the earliest form of breast cancer. Abnormal cells are located inside milk ducts in the breast and have not spread or invaded other parts of the breast. Its detection has significantly increased since routine mammographic screening. The management of DCIS includes surgical intervention and with radiotherapy as appropriate. Chemoprevention may be used in people who have been treated for DCIS to prevent the development of breast cancer. The most commonly used chemoprevention is with hormone therapy involving oestrogen receptor (ER) blockers (tamoxifen or raloxifene) or aromatase inhibitors (AIs; anastrozole, exemestane and letrozole). These hormone therapies are an established treatment for women with ER-positive invasive breast cancer. At the time of the previous guideline CG80 (NICE 2009), evidence was felt to be conflicting around use of hormonal therapies (chemoprevention) after adequate surgical treatment of DCIS. The aim of this review is to assess the role of chemoprevention in women with DCIS, which will consider the benefits of reducing breast cancer recurrence and secondary breast cancers, compared to the side effects of increased risks of endometrial cancers and thromboembolic complications.