The relationship between bilingualism and cognitive development is explored as an exemplary area in which the disciplinary concerns of cognitive psychology, social psychology, and sociology occur together. A historical review of research shows that many of the apparently contradictory findings about the effects of bilingualism on mental development have stemmed from a failure to distinguish between levels of bilingualism as defined by the three different research orientations. The literature within each of the disciplines is discussed and the implications for a more rigorous definition of bilingualism are outlined, based on research undertaken in a bilingual education program in New Haven, Connecticut. Sixty-nine references are listed. (Author/MSE)