Empirical and theoretical research on language has recently experienced a period of extensive growth. Unfortunately, however, in the case of the Japanese language, far fewer studies—particularly those written in English—have been presented on adult second language (L2) learners and bilingual children. As the field develops, it is increasingly important to integrate theoretical concepts and empirical research findings in second language acquisition (SLA) of Japanese, so that the concepts and research can be eventually applied to educational practice. This article attempts to: (a) address at least some of the gaps currently existing in the literature, (b) deal with important topics to the extent possible, and (c) discuss various problems with regard to adult learners of Japanese as an L2 and English–Japanese bilingual children. Specifically, the article first examines the characteristics of the Japanese language. Tracing the history of SLA studies, this article then deliberately touches on a wide spectrum of domains of linguistic knowledge (e.g., phonology and phonetics, morphology, lexicon, semantics, syntax, discourse), context of language use (e.g., interactive conversation, narrative), research orientations (e.g., formal linguistics, psycholinguistics, social psychology, sociolinguistics), and age groups (e.g., children, adults). Finally, by connecting past SLA research findings in English and recent/present concerns in Japanese as SLA with a focus on the past 10 years including corpus linguistics, this article provides the reader with an overview of the field of Japanese linguistics and its critical issues.