This paper is concerned with the inner speech process, its relationship to thought and behavior, and its theoretical and educational implications. The paper first defines inner speech as a bridge between thought and written or spoken language and traces its development. Second, it investigates competing theories surrounding the subject with an emphasis on the theories of Lev Vygotsky. Both laboratory and documentary evidence of the existence of inner speech is presented, and a pilot study in which marginal notes supplied by students or randomly selected from library books were classified according to the three categories suggested by James Wertsch is discussed. Lastly, the paper examines the significance of inner speech as well as its educational implications. Two figures and one table are included; 15 references are attached. One appendix provides samples of marginal notes from the pilot study. (MG)