Antioestrogens (AEs) are synthetic molecules that block proliferation and induce apoptosis in breast cancer (BC) cells, principally by competing with oestradiol for binding to oestrogen receptors. Their antiproliferative activity and their pro-apoptotic capacity are well documented and a small number of molecules of this class are currently used clinically for the treatment of BC. AEs also inhibit cell cycle progression and/or induce apoptosis in multiple myeloma (MM) cells. Encouraging preliminary results have been obtained with patients and on xenografts with MM, providing a rational basis for the clinical use of AEs. This review focuses on antioestrogen-mediated signalling for blocking targets involved in the cell cycle, survival and apoptosis in both BC and MM cells. Improvement in our understanding of the mechanisms underlying the relationships between these compounds and their targets should lead to more beneficial therapeutic strategies.