Despite the wealth of sequence data available from human, mouse and other genomes, our understanding of the mechanisms involved in regulating transcription and creating protein diversity is incomplete. Although effects such as alternative splicing have been extensively studied, other less well-characterized phenomena are now being recognized that increase the complexity of the transcribed portion of the genome. One mechanism, supported by a growing body of research, is the use of alternative promoters. In this article, we describe the consequences and significance of alternative promoter usage in human and mouse genomes, demonstrating that mammalian genes commonly employ multiple promoters to regulate and increase their transcriptional and translational potential.