The goal of this chapter is to present a theory of bilingual spelling in alphabetic systems (BAST). This theory is derived from dual-process models of monolingual spelling and integrates key representational and processing assumptions from research on bilingual spoken word production. The aim of the theory is to provide an account of the processes that underlie single word spelling in bilinguals. Mechanisms are proposed for spelling both familiar and unfamiliar words. In addition, the theory includes hypotheses about how and when information from the two languages may interact. The framework makes specific predictions about spelling performance in bilingual individuals, in particular about the way it may be influenced by interactions between the two languages, by word characteristics, and by the degree and type of bilingualism. Some of these predictions are examined by presenting experimental evidence from the spelling performance of healthy and neurologically impaired bilingual adults. The chapter concludes by discussing how BAST could further contribute to broaden our understanding of bilingual spelling in different populations.