This book is an investigation of an unprecedented learner corpus systematically collected from learners of Japanese as a second or foreign language in 20 areas of the world. It addresses the issues of second language acquisition in general, and of Japanese in particular. In the 50s and 60s the main factor underlying the errors committed by foreign language learners was considered to be `language transfer.' In the 70s it became increasingly clear that learners with different L1s commit similar mistakes in the acquisition of a given L2. It was proposed that the main source of the errors was not `language transfer' but a potentially universal `inter language' which learners construct on their transition trajectory from L1 to L2. In the 80s various other approaches such as `universal grammar', `monitor model', `competition model' evolved, all aiming to account for the errors of L2 learners. Against this backdrop, this proposed volume offers new insights to the theory of language acquisition by focusing on a major Asian language, viz Japanese, and makes a significant contribution to the theory of language acquisition through an investigation of empirical data of learners of Japanese with as many as 12 different L1s and varied typological profiles.