Transcription factors are key players in the execution of the genomic program of any given cell. Their control over complex patterns of gene expression governs essentially every step in the development, growth and differentiation of an organ ism as well as its physiological responses to external stimuli. An Introduction to Transcription Factors, the first part of this volume, de scribes the varied and combinatorial mechanisms involved in the two basic modes of transcriptional control, activation (the chapter by Herrera et al.) and repression (the chapter by Arnosti). Overlaying both modes is...