Accurate transcription of eukaryotic genes in vitro is preceeded by the assembly of the template DNA into stable DNA-protein complexes. Such complexes have been reported for genes transcribed by each of the three eukaryotic RNA polymerases. DNAase I protection or footprint assays have yielded information as to the sites of protein factor binding. These sites correlate with many of the sequences which have been implicated as promoter elements through analysis of deletion mutants. Stable transcription complexes are also formed in microinjected Xenopus oocytes and such complexes can be shown to exist in vivo in nuclei and chromatin. The propagation of active transcription complexes may prove to be a crucial element in cellular differentiation.