A discussion of the nature and function of interlanguage in second language acquisition draws on research on interlanguage and experiences in teaching German to Finnish secondary students. The theories giving rise to the concept of interlanguage are outlined and compared, and the characteristics of interlanguage are discussed. Special attention is given to the roles of grammar, interpretation, interlanguage as dialect, linguistic patterns of the language learner, psycholinguistic processes in language learning, and fossilization. Grammatical and other language patterns reflecting the development of interlanguage found in 432 German essays written by 38 Finnish students are briefly described, and questions to be addressed in future research about interlanguage are presented. A 31-item bibliography is included. (MSE)