A general picture of the role of expression divergence in the evolution of duplicate genes is emerging, thanks to the availability of completely sequenced genomes and functional genomic data, such as microarray data. It is now clear that expression divergence, regulatory-motif divergence and coding-sequence divergence all increase with the age of duplicate genes, although their exact interrelationships remain to be determined. It is also clear that gene duplication increases expression diversity and enables tissue or developmental specialization to evolve. However, the relative roles of subfunctionalization and neofunctionalization in the retention of duplicate genes remain to be clarified, especially for higher eukaryotes. In addition, the relationship between gene duplication and evolution of transcriptional regulatory networks is largely unexplored.