Welcome to the American Journal of Play special issue on the science of play, the first of a number of theme issues that will appear from time to time. Each will focus on an important topic in the fast-developing study of play. Each will be guest edited by a distinguished expert on the topic. And each will include work by the leading researchers and thinkers on the topic. In this issue, guest-editor Stephen M. Siviy has assembled a series of articles on the scientific study of play as pioneered by Jaak Panksepp (who is interviewed in the issue). These include a seminal article on laughing rats written by Panksepp and Jeffrey Burgdorf and reprinted here for ease of reference as a historical backdrop to the other pieces in this issue. They begin with an article written by Sergio M. Pellis, Vivien C. Pellis, and Heather C. Bell on the subcortical brain mechanisms involved in play. Our guest editor himself contributes a piece exploring the impact of fear and anxiety on play in laboratory animals. Louk J. M. J. Vanderschuren investigates how the brain makes play fun. And Gordon M. Burghardt reviews the growing information on play in species other than our own. A topic not without its controversy, the science of play has been reshaping the landscape for our understanding of play—where it comes from and how it affects us.