Estrogens primarily function through the activation of their receptors, which subsequently function as nuclear transcription factors. There are two estrogen receptor (ER) genes, now designated ERα (the classic ER gene) and ERβ. The key consequence of the activation of either gene product is the regulation of gene transcription. The extent and nature of transcription appear to be regulated by a series of coregulator proteins. One of the most sensitive assays for detection of potential estrogenic activity is measurement of the ability of a test compound to influence the transcription of reporter genes. In this regard, many investigators use promoter-reporter constructs. To assess putative estrogenic activity, an estrogen-responsive promoter is generally placed upstream of a reporter gene and transiently transfected into a target cell. When exposed to an estrogenic compound, expression of the reporter gene would normally be induced. We briefly discuss several issues pertinent to the use of these assays and the interpretation of resulting data, including estrogen-responsive, promoter-reporter constructs, reporter genes and measurements of activity, choice of target cell or cell line, transient introduction of promoter-reporter constructs into cells, basic statistical approaches to data analysis, and definitions of agonist, partial agonist, and antagonist.