Progestins play a key role in reproductive endocrinology and as pharmaceutical drugs for contraception and in combined hormone therapy. To further our understanding of progestin action in the mammary gland, an international symposium, attended by leading researchers from academia and industry, was held in Berlin, 21 23 March 2007. Genetic mouse models helped to elucidate the role of progestins, both in normal breast development and in disease. Mechanistic molecular studies inspired the design of new progestins with improved tissue selectivity. In addition, the clinical impact of progesterone receptor agonists and antagonists for the prevention and treatment of breast cancer was discussed.