The routine isolation and culture of human breast cancer cells from patient samples has been a goal of breast cancer cell biologists for over 30 years. Despite extensive work in this area and the development of many human breast cancer cell lines, the proportion of patient samples that give rise to immortalized breast cancer cell lines is still disappointingly low. The majority of human breast cancer cell lines that have been established were isolated many years ago and have been grown continuously under poorly defined culture conditions. These cell lines have been useful for studies of the estrogen receptor biology in human breast cancer cells, in identifying growth factors synthesized by breast cancer cells, and for the characterization of genetic alterations in oncogenes and tumor suppressor genes present in these cells. More recently, tissue culture methods have improved, resulting in the ability to culture routinely normal human mammary epithelial cells of specific lineages and this has resulted in the development of new human breast cancer cell lines. The ability to isolate and culture normal and neoplastic human mammary epithelial cells under similar culture conditions has improved these models dramatically and has resulted in the identification of altered cellular phenotypes of human breast cancer cells.