This chapter reviews the studies about koine, a linguistic variety that emerged from contact between dialects. Research in the western and Japanese contexts are introduced as follows: (1) in the western research context, the term “koine” originally referred to a variety of Greek during the Hellenistic and Roman ages. In western sociolinguistics, early research was launched on varieties similar to the Greek koine. Later, two types of koine were distinguished, one is a regional koine, the other is immigrant koine. In these studies, developmental mechanisms of koine were conceptualized; (2) in the Japanese context, the discussion focuses on immigrant koine. It deals with linguistic varieties that emerge in new communities by migration from several regions: the cases of the Ogasawara (Bonin) Islands and of “Seishin New Town” are reviewed. Though many examples of contact of Japanese dialects overseas and in Japan have been identified, most of them have not been studied as koine. This chapter concludes with three suggestions. First, many cases of Japanese varieties should be studied as koine. Moreover, the concept of koine may be expanded on contact between not only regional but also social dialects. Finally, the relationship between dialect and koine should be clarified.