Medline Plus
http://www.nlm.nih.gov/medlineplus/ency/article/001523.htm
Education.com
http://www.education.com/reference/article/characteristics-children-mental-retardation/
Council for Exceptional Children
http://www.cec.sped.org/am/template.cfm?section=Home
Bright Hub
http://www.brighthub.com/education/special/articles/73681.aspx
