Based on preclinical studies and early clinical observations, an association between vitamin D status and breast cancer incidence and outcome has been proposed. Against this background, information on vitamin D and breast cancer was reviewed with focused attention on emerging clinical studies in this area. Prospective cohort studies do not associate 25-hydroxyvitamin D levels with breast cancer incidence. While case-control studies of this question are positive, they may be confounded by reverse causality as 25- hydroxyvitamin D levels are influenced by breast cancer presence and stage. Studies of 25-hydroxyvitamin D and subsequent breast cancer recurrence provide mixed results but strongest associations were seen in analyses uncontrolled for prognostic variables, cancer therapy, BMI and physical activity. The one full-scale randomized, placebo-controlled trial evaluating calcium (1000 mg elemental calcium per day) and vitamin D supplementation (400 IU D3 per day) with 36,282 participants failed to demonstrate a supplement effect on lowering breast cancer incidence. Breast cancer patients not uncommonly have vitamin D deficiency but limited control populations in available reports preclude precise prevalence estimates. As breast cancer patients are at risk for bone loss and musculoskeletal complaints from cancer or associated therapies, monitoring 25-hydroxyvitamin D levels and vitamin D3 supplementation in moderate dose (1,000- 1,500 IU D3 per day) can be recommended with expectation of mainly bone benefit. In women with breast cancer, future vitamin D supplementation studies need to be appropriately designed and powered to provide definitive assessments. However, a full-scale randomized trial evaluating the influence of vitamin D supplementation on breast cancer recurrence is likely not feasible.