Drawing on the perspective of language socialization and a theory of indexicality, this book explores ways in which learners of Japanese as a foreign language and their Japanese host families socialize their identities through style shift between the masu and plain forms in a homestay context. Going beyond the usual assumption that the masu form is a polite speech marker, the book analyzes the masu form as an index of various social identities and activities. The book discusses both socialization through speech styles and socialization to use an appropriate speech style. Qualitative analysis of dinnertime conversations demonstrates how learners are implicitly and explicitly socialized into the norms of style shift in Japanese in interaction with their host family members.