At least half of all advanced breast cancers are positive for estrogen receptor (ER) and progesterone receptor (PR), but many nevertheless fail to respond to endocrine therapy. Studies of breast cancer cell lines and breast tumor specimens are beginning to reveal molecular heterogeneity of the receptors in subpopulations of these cells, leading to altered receptor function and sometimes to hormone resistance. Here we will review the data on molecular and cellular heterogeneity involving ER and PR, and possible underlying mechanisms of resistance to tamoxifen and progestins.