Within the past few years, the genes encoding transcription terminator proteins for RNA polymerase I (pol I) have been cloned from organisms as diverse as yeast and mammals. The availability of terminator proteins has allowed construction of in vitro transcription systems that terminate pol I at the same sites as used in vivo and thus allows study of termination mechanisms. This has resulted in a burst of information concerning pol I termination mechanisms, which this review will attempt to summarize.