The regulation of gene expression in prokaryotic cells occurs at the transcriptional level. Prokaryotes only transcribe genes that their end-proteins are needed at the time. The regulation of gene expression is depended mainly on their immediate environment, for example on the presence and absence of nutrients. Control of gene expression is at the level of transcription. If a gene is not transcribed then the gene product and ultimately the phenotype will not be expressed. This book attempts to present some of the recent work on regulation of gene expression in prokaryotic cells valuable to students and practitioners dealing with the subject of biochemistry, molecular biology, and microbiology will also find this book useful.