A contemporary view of hormone action at the transcriptional level requires knowledge of the transcription factors including the hormone receptor that may bind to promoters or enhancers, together with the chromosomal context within which these regulatory proteins function. Nuclear receptors provide the best examples of transcriptional control through the targeted recruitment of large protein complexes that modify chromosomal components and reversibly stabilize or destabilize chromatin. Ligand-dependent recruitment of transcriptional coactivators destabilizes chromatin by mechanisms including histone acetylation and contacts with the basal transcriptional machinery. In contrast, the recruitment of corepressors in the absence of ligand or in the presence of hormone antagonists serves to stabilize chromatin by the targeting of histone deacetylases. Both activation and repression require the action of other chromatin remodeling engines of the switch 2/sucrose nonfermentable 2 (SWI2/SNF2) class. Here we summarize this information and integrate hormone action into a chromatin context. Journal of Molecular Endocrinology (1999) 23, 255-275 INTRODUCTIONThe nuclear hormone receptors provide transcription research with a clear example of how the reversible modification of chromatin structure can contribute to the control of gene expression. Remarkable progress in the definition of intermediary protein complexes that activate or repress transcription has allowed common themes in transcriptional control by a wide variety of receptors to emerge. The roles of these coactivators and corepressors in mediating the activities of nuclear receptors within chromatin is a key element of contemporary molecular endocrinology. The beststudied receptors in this regard are those for the glucocorticoid and thyroid hormones.The glucocorticoid receptor recognizes response elements within nucleosomes as a first step towards a finely orchestrated rearrangement of histone-DNA contacts concomitant with the assembly of a functional transcription complex. The determinants of chromatin remodeling and the molecular machines that carry it out are now understood in considerable detail. Likewise, the thyroid hormone receptor recognizes nucleosomal DNA and utilizes chromatin to regulate transcription. However, in this case the receptor exerts a dual function. In the absence of ligand, the thyroid hormone receptor recruits a corepressor complex that stabilizes chromatin structure. Upon addition of hormone the receptor releases this repressive complex, and recruits coactivators that destabilize chromatin and promote transcription.This review illustrates the role of chromatin, coactivators and corepressors in gene control as orchestrated by the glucocorticoid and thyroid hormone receptors. COACTIVATORS AND COREPRESSORS CoactivatorsThe interplay of distinct nuclear receptor responsive transcription pathways has been recognized for 255