Language contact is at its most intense within one and the same individual. This chapter discusses the dynamic and multifaceted phenomenon of individual bilingualism, which emerges when individuals learn to understand multiple language varieties. Individual bilingual language use may contribute to societal processes of language change, language maintenance, and language loss. It is as yet not fully clear how bilingual individuals affect such larger processes, but cross-linguistic influence in comprehension and production, patterns of language choice, and variable levels of proficiency across the lifespan all play a role. These in turn depend on individuals' contexts for learning and using languages throughout the lifespan. The chapter exemplifies some of these on the basis of bilinguals' language biographies (including Frederick the Great's). Language learning histories and opportunities for using each language help explain the large variability in language skills and use among bilinguals. New languages can be learned until well into adulthood. Their number is constrained only by learning opportunities and motivation. Previously learned languages, including languages learned very early in life, can be lost through lack of use and practice. Language attitudes play a large role in all of this.