Bioactive phytochemicals are continually being studied for their role in cancer prevention with increasing evidence for flavonoids, carotenoids, phenolic acids, and organosulfur compounds (found in cruciferous vegetables). This 2018 review explores the protective effects of a broad spectrum of plant-derived substances. In total, more than 5000 individual phytochemicals have been identified in plant-derived foods, such as fruits, vegetables, and grains. These bioactive compounds have been shown to have antitumor activity, reduce inflammation, induce apoptosis (cell death), inhibit the proliferation of aggressive tumour cells, and impact on metastasis (migration of cancer cells). Specifically, in breast cancer, a few studies have examined phytochemicals on cancer stem cells (the originating tumour cells) and found that curcumin, genistein, indol-3-carbinol, c-phycocyanin, resveratrol, and quercetin downregulated their activity. Systematic reviews of dietary patterns and breast cancer show vegetables, and especially fibre, to be consistently protective against reduced risk of mammary carcinogenesis. Dietary polyphenols are considered a cost-effective approach to cancer care however there is still a lack of evidence due to the complex nature of combined phytochemicals versus isolated agents. Wholefood consumption is considered to improve bioavailability compared to supplementation however phytochemicals are a low-dose component of foods. There is also concern that some phytochemicals may act as carcinogens or tumour promoters (for example, beta-carotene). More clinical trials are required to fully understand phytochemicals and breast cancer care.