Abstract: The aim of this article is to explore the differences in nature and origin between two constructs that are used in the contemporary professional literature to explain the processes and consequences of language learning: i (input) + 1 and the Zone of Proximal Development (ZPD). Each construct is situated with respect to its history and to the fundamental assumptions it makes about language, the mind, and society. A brief comparison is established between the ways each construct is used to interpret the role of social interaction in the classroom. Within this comparison, particular emphasis is placed on the implications of the ZPD, because this construct is the least familiar of the two within the professional discourses of language education in the United States.