Knowledge of the structure of ancestral genomes provides the basis of a new framework to better represent and interpret results from genomic and evolutionary studies. Because these ancestors lived tens of hundreds of million years ago, this knowledge will inevitably take the form of abstract representations, reconstructed on the basis both of experimental evidence collected on extant genomes and of our understanding of evolutionary processes. This is the field of Paleogenomics, a young discipline that is providing an increasingly precise picture (...) of our ancestral vertebrate genomes based on cytogenetic data, genome sequences and new algorithmic developments. Many recent studies have focused on the ancestral placental mammal and teleost fish genomes, although the outlines of even more distant pre‐vertebrate ancestors are being reported. BioEssays 30:122–134, 2008. © 2008 Wiley Periodicals, Inc. (shrink)