Gilbert Holder, the Brand and Monica Fortner Endowed Chair in Theoretical Astrophysics at the University of Illinois, is a theoretical astrophysicist and cosmologist who seeks to understand how the complex universe we see today--full of galaxy clusters, galaxies, stars, and planets--evolved from a nearly uniform state having density fluctuations of a few parts per million 300 000 years after the big bang. Professor Holder has worked on a wide range of topics--from the early universe, to the cosmic microwave background, to gravitational lensing, to black holes, to our solar system. He has developed theoretical methods and tools to investigate these astronomical objects, enabling important constraints on the properties of dark matter and dark energy.