A fundamental principle of exponential bacterial growth is that no more ribosomes are produced than are necessary to support the balance between nutrient availability and protein synthesis. Although this conclusion was first expressed more than 40 years ago, a full understanding of the molecular mechanisms involved remains elusive and the issue is still controversial. There is currently agreement that, although many different systems are undoubtedly involved in fine-tuning this balance, an important control, and in our opinion perhaps the main control, is regulation of the rate of transcription initiation of the stable (ribosomal and transfer) RNA transcriptons. In this review, we argue that regulation of DNA supercoiling provides a coherent explanation for the main modes of transcriptional control - stringent control, growth-rate control and growth-phase control - during the normal growth of Escherichia coli.