The biological actions of estrogens are mediated by estrogen binding to one of two specific estrogen receptors (ERs) ERalpha and ERbeta, which belong to the nuclear receptor superfamily, a family of ligand-regulated transcription factors. ERalpha and ERbeta are products of different genes and exhibit tissue- and cell-type specific expression. The characterization of mice lacking ERalpha, or ERbeta, or both has revealed that both receptor subtypes have overlapping but also unique roles in estrogen-dependent action in vivo. Additionally, ERalpha and ERbeta have different transcriptional activities in certain ligand, cell-type, and promoter contexts. Both receptors, however, are coexpressed in a number of tissues and form functional heterodimers. The biological roles of ERalpha /beta heterodimers in the presence of each respective homodimer are unknown. When coexpressed, ERbeta exhibits an inhibitory action on ERalpha -mediated gene expression and in many instances opposes the actions of ERalpha. A number of ERalpha and ERbeta isoforms have also been described, many of which alter estrogen-mediated gene expression. Uncovering the molecular mechanisms regulating the expression of both ERs, and how ERalpha and ERbeta directly or indirectly affect each other's function are paramount to understanding the cellular and biological events of estrogen-mediated gene regulation in normal and diseased tissues.