Development, differentiation and response to environmental stimuli are characterized by sequential changes in cellular state initiated by the de novo binding of regulated transcriptional factors to their cognate genomic sites^,,. The mechanism whereby a given regulatory factor selects a limited number of in vivo targets from a myriad of potential genomic binding sites is undetermined. Here we show that up to 95% of de novo genomic binding by the glucocorticoid receptor, a paradigmatic ligand-activated transcription factor, is targeted to preexisting foci of accessible chromatin. Factor binding invariably potentiates chromatin accessibility. Cell-selective glucocorticoid receptor occupancy patterns appear to be comprehensively predetermined by cell-specific differences in baseline chromatin accessibility patterns, with secondary contributions from local sequence features. The results define a framework for understanding regulatory factor–genome interactions and provide a molecular basis for the tissue selectivity of steroid pharmaceuticals and other agents that intersect the living genome.