Nuclear hormone receptors form one evolutionary related super-family of proteins, which mediate the interaction between hormones (or other ligands) and gene expression in animals. Early phylogenetic analyses showed two main periods of gene duplication which gave rise to present-day diversity in most animals: one at the origin of the family, and another specifically in vertebrates. Moreover this second period is composed itself by, probably, two rounds of duplication, as proposed by Susumu Ohno at the origin of vertebrates. There are indeed often two, three or four vertebrate orthologs of each invertebrate nuclear receptor, in accordance with this theory. The complete genome of Drosophila melanogaster contains 21 nuclear receptors, compared to 49 in the human genome. In addition, many nuclear receptors have more paralogs in the zebrafish than in mammals, and a genome duplication has been proposed at the origin of ray-finned fishes. Nuclear receptors are a very good model to investigate the dating and functional role of these duplications, since they are dispersed in the genome, allow robust phylogenetic reconstruction, and are functionnaly well characterized, with different adaptations for different paralogs. We illustrate this with examples from differents nuclear receptors and different groups of species.