Steroid hormones such as estrogens are known to signal through ligand-regulated transcription factors of the nuclear receptor superfamily. In addition they elicit rapid non-genomic responses from membrane-associated receptors. One of these receptors belongs to an entirely different family of proteins. The G-protein coupled and seven transmembrane receptor GPR30 is now widely recognized as an estrogen receptor, hence its official new acronym GPER. It appears to mediate a wide range of responses to estrogen in a large variety of cell types. Its functions are clearly distinct from those of the classical nuclear estrogen receptors, although these pathways may overlap and interact in some cases. Here we review the history of the discovery of this new estrogen receptor, the evidence for the claim that it is an estrogen receptor, its signal transduction, and its potential functions in physiology and disease.