While not common in books written for the medical professional, the encyclopedia format has long been popular as a means of providing medical information to the layman. Such august institutions as the Johns Hopkins University, Cornell University, Rush-Presbyterian-St. Luke’s Medical Center, and the American Medical Association publish health encyclopedias of various sorts, and there are a grand total of 768 listings under the term medical encyclopedia on the Barnes & Noble Web site. Even the Disney Company publishes an encyclopedia of pediatric medicine! Given the public’s increasing familiarity with sleep disorders, it is not surprising that an encyclopedia pertaining to this field would appear. In fact, there are two: the volume by Thorpy and Yager that is the subject of this review, and Mary Carskadon’s Encyclopedia of Sleep and Dreaming, first published in 1993 and reissued in 1996.