The mammary fat pad is essential for developmentof the mammary epithelium, providing signals thatmediate ductal morphogenesis and, probably, alveolardifferentiation. The “cleared” fat pad is often used as a transplantation site.Considering the crucial role of the fat pad, itsproperties have received relatively little attentionfrom researchers in the field. Some of the questionswhose investigation is pertinent to understanding both normalmammary development and carcinogenesis are outlined inthis commentary in the spirit of stimulating enquiryinto this important subject. It is clear from a brief perusal of the available literature that untilstudies are specifically designed to clearlydifferentiate between functional effects of the fibrousand the adipose stroma, more substantive informationabout their differential effects on mammarydevelopment and tumorigenesis will not beforthcoming.