Within an individual organism, every cell contains essentially identical genomic sequence. How then do cells develop and function differently from each other? The answer lies in the regulation of gene expression. Only certain genes are expressed (i.e. are functionally active) under any particular biological circumstances. Gene expression is regulated at many different stages of the process that converts DNA information into active protein. In the first stage, transcript abundance can be controlled by regulating the rate of initiation of transcription, and the processing and degradation of transcripts. In many cases, higher abundance of a gene’s transcripts is correlated with its increased expression. In this chapter, we will focus on transcriptional regulation, but be aware that cells also regulate the activity of genes in other ways. For example, by controlling the rate of translation, processing, degradation, and post-translational modification of proteins and protein complexes.