Interlanguage (IL) is the learner's language which is in the process of transition between the learner's native language (NL) and the target language (TL). IL is characterized by systematicity and variability. By analyzing IL we can obtain information about learner's strategies and processes in acquiring the TL. This paper introduces some studies which have been conducted on strategies, phonological developments, and other factors in Japanese learners' IL: acquisition of Japanese geminate consonants by Australian learners, voiced stops by Beijing and Shanghai Chinese learners, compensatory lengthening by British learners, auditory recognition of phrasal pitch patterns by Korean learners from Seoul, and production of different phrasal pitch patterns by French learners. The results from the IL research may provide suggestions to help learners of Japanese as well as better understanding of language learning in general.
