Idioglossia is a private communication system, most commonly occurring in twins. It also occurs between singletons and between other siblings of multiple births. These communication systems range from manual gestures to a fully developed vocal language with its own grammar. The literature of idioglossia is scanty and largely anecdotal. Much of the literature implies that idioglossia is a rare pathology. However, none of the studies on language retardation in twins has tested the relationship between the use of "twin" language and continuing academic retardation. Idioglossia also may not be so rare. Some studies report that 40 to 50 percent of all twins use "secret language." In the author's informal survey, idioglossia was found in six out of seven sets of twins plus one set of singleton siblings. Hints about the circumstances which lead to the development of idioglossia can be discovered from the Kennedy twins, who have persisted in using idioglossia to the near exlusion of other languages. The development of idioglossia also may have implications for issues in linguistics (i.e., the innateness of language and Sapir-Whorf theory). There is an astounding gap between folk knowledge of the phenomena and scholarly research which implies that much more research is needed. (CB)