Although all cells in a multicellular organism carry the same genes, they become specialized cells with different functions, regulated by the unique expression of the genes they carry. This chapter introduces the subject of prokaryotic and eukaryotic regulation of gene expression. Prokaryotes principally turn their genes on and off using activators and repressors, whereas gene expression in eukaryotes involves many regulatory steps, due to their DNA structures and cellular compartmentalization. In eukaryotic cells, the major point of regulation occurs at the initiation of transcription, through chromatin modifications and transcription factor interactions. Unique structural motifs of various transcription factors and epigenetic control of gene expression are also discussed in the chapter.