Lexical semantics and contrastive lexical semantics can serve as a background discipline to describe and, to some extent, to explain errors in interlanguage. Two developments in lexical semantics that are relevant in this area are the description of sense-relationships and componential analysis. Contrastive lexical semantics involves mapping the meanings of second language items onto those of equivalent items in the native language, and more broadly, comparing, in lexical terms, the ways the two languages organize perception of the world. Contrastive lexical semantics is essential for understanding some aspects of lexical difficulty. Knowledge in these areas can give insight into the ways a second language learner semanticizes, presents, and selects vocabulary, the ways in which vocabulary is structured, the sorting out of meanings in closely-knit semantic fields, the relatedness of lexical items from different semantic fields, and the factors affecting word association, all of which relate in some way to the development of interlanguage. These are factors that must be kept in mind when one is designing materials and techniques for teaching and learning a second language. (MSE)