It is difficult to overstate the evolutionary and functional significance of mammary tissue in biology. Substantial progress has been made by researchers in various disciplines towards realizing the potential of this tissue to yield powerful experimental models for morphogenesis and tissue development; for cellular differentiation; for the biosynthesis and secretion of proteins, lipids, small molecules and inorganic salts; and for the coordination and regulation of these processes. Also, the possibility of exploiting the secretory epithelial cells of mammary tissue as "cell factories" has become a reality and the recombinant production by lactating animals of an increasing number of proteins, valuable both in the pharmaceutical and "nutraceutical" fields, is in progress or under development. This volume provides a glimpse into our understanding, at the cutting edge of a variety of disciplines, of this versatile and extraordinary tissue, at the birth of the 21st century.