A detailed knowledge of the mechanisms underlying the transcriptional control of gene expression is of fundamental importance to many areas of contemporary biomedical research, ranging from understanding basic issues (such as control of embryonic development) to practical applications in industry and medicine. Although elementary concepts of gene expression are described in all general molecular biology textbooks, the depth of coverage is often rather limited and recent discoveries are sometimes not adequately taken into consideration.This book presents much of the current thinking concerning molecular mechanisms of transcriptional control in a form easily accessible to undergraduates with an understanding of basic molecular biology concepts. It contains detailed information about the various pro- and eukaryotic transcriptional machineries that has recently become available through the combined efforts of geneticists, biochemists and structural biologists. The book will thus not only serve as an undergraduate text but also offer something new and interesting to more advanced readers and professional scientists who want to keep up to date with rapid advances in this field.