Currently, breast cancer is considered as one of the leading causes for death in women in the United States. Consumption of natural products has received considerable attention in recent years as a possible approach for cancer prevention in general population. There are numerous cancer preventive agents present in the natural products, which may contribute to their chemopreventive properties. During the past two decades, numerous chemopreventive agents have been isolated and/or synthesized and evaluated for their efficacy in a variety of biological assays. To this end, we have established and utilized mouse mammary gland organ culture model (MMOC) as a bioassay for identifying chemopreventive agents. Mammary glands respond to growth promoting hormones and the physiological differentiation can be reproduced in MMOC in chemically defined medium by altering hormonal milieu. Both estrogen and progesterone dependent (mammary ductal lesions, MDL) and independent (mammary alveolar lesions, MAL) precancerous lesions can be induced in response to a 24 hour exposure to DMBA in MMOC. Suppression of the incidence and multiplicity of these lesions by a possible chemopreventive agent can serve as a tool to evaluate efficacy of potential experimental agents. Using this approach, we have evaluated more than 200 synthetic and natural product-derived chemopreventive agents in this model as a part of the National Cancer Institute-supported projects. Many of these chemopreventive agents expressing significant activity have progressed to the in vivo experimental mammary carcinogenesis studies. Thus, this bioassay has proven to be a valuable tool for screening cancer chemopreventive agents for breast cancer prevention and for understanding molecular mechanism(s) of action of these agents. In this comprehensive review, we provide a complete list of chemopreventive agents evaluated for the efficacy against development of mammary alveolar lesions (MAL) in MMOC along with the recent developments in this area. The structure-activity relationships for many chemopreventive agents evaluated in the MMOC model have been discussed.