Why an encyclopedia of autism? There are several answers to this question. They include the need to provide a comprehensive and current guide to the diverse knowledge now available. There has been a significant upsurge in research in autism during the past two decades. Several hundred papers were published in 1991 compared to more than 2,000 articles during 2011. The quantity of research (not even counting non-peer-reviewed publications) has increased so dramatically that it is difficult, if not impossible, for researchers and clinicians to keep up. Access to a reference work that provides an introduction to relevant information is clearly needed. Although several excellent handbooks and textbooks have been published in recent years, these are, almost intrinsically, fated to become increasingly out of date more and more quickly. Fortunately, many of the same technological advances that have been adapted for use with individuals with autism have uses for those of us who support them. The ability to produce both a print reference work as well as an online version with additional content was a major attraction for us in undertaking this project. It also can be updated easily and will have additional content. The electronic format also provides for an extensive cross-referencing system, which is designed to facilitate rapid searching and information retrieval. With contributions on a range of topics from leaders in the field, this reference work breaks new ground as a resource. The Encyclopedia contains several thousand entries relevant to autism and related conditions, including new research findings; entries on development and behavior; assessment methods and instruments; treatments and educational interventions; biographies of leaders in the field; and information relevant to epidemiology, social policy, and treatment planning. Both I and the associate editors of this work hope that you will benefit from using the encyclopedia and welcome your feedback. By the time the print publication of this work appears, the online edition will already have had entries added reflecting new knowledge in various areas. We hope that this resource enhances the work of clinicians and researchers alike.