This special issue of The Anatomical Record originates from a symposium on the evolution of neurobiological specializations in mammals held at the American Association of Anatomists annual meeting in San Diego in April 2005. The symposium, co-organized by Patrick R. Hof and Lori Marino, provided the impetus for extending the discussion to a greater range of species. This special issue is the product of that goal and is fueled by the philosophy that it is largely against a backdrop of brain diversity that we can extract the higher-order commonalities across brains that may lead us to uncovering general higher-order principles of brain and behavioral evolution. Several major themes emerge from this issue. These are that there are no simple brains, that brains reflect ecology, and that brain evolution is a detective story. The 12 articles in this issue are outstanding reflections of these themes. © 2005 Wiley-Liss, Inc.