Understanding the immune defenses of the mammary gland is instrumental in devising and developing measures to control mastitis, the major illness of dairy ruminants. Innate immunity is an extremely broad field for investigation, and despite decades of research, our present knowledge of the innate defenses of the udder is incomplete. Yet, information is being gained on the recognition of pathogens by the mammary gland, and on several locally inducible defenses. The contribution of mammary epithelial cells to local defenses and to the mobilization of leucocytes is under growing scrutiny. Interactions of mastitis-causing bacteria such as Escherichia coli or Staphylococcus aureus and the mammary gland represents a suitable model for studies on innate immunity at an epithelium frontier. Powerful new research tools are radically modifying the prospects for the understanding of the interplay between the mammary gland innate defenses and mastitis-causing bacteria: genetic dissection of the immune response, microarray gene technology, transcriptomic methodologies and gene silencing by RNA interference will make possible the discovery of several of the key defense mechanisms which govern the susceptibility/resistance to mastitis at the molecular and genetic levels. It should then be possible to enhance the resistance of dairy ruminants to mastitis through immunomodulation and genetic improvement.