This article explores major processes that operate in new dialect formation, with a focus on the effects of various kinds of isolation of communities and their speakers. A sociolinguistic approach concentrates on competition and selection between transplanted dialect features and mechanisms such as mixing, leveling, simplification, and reallocation. With reference to the established (yet not uncontroversial) concept of the feature pool, the question is how contact between distinct systems gives rise to localized forms. From a more socially oriented perspective, the question is how the integration and segregation of groups contributes to or shapes the emergence and disappearance of dialects. Isolation as a sociolinguistic concept is not thoroughly defined, particularly at the intergroup, societal, or even national level; therefore, geographic, social, and psychological factors need to be discussed and assessed. The processes investigated here are koinéization and dialect/language shift, illustrated with examples from all over the world.