‘Gender’ is a manifold notion, at the crossroads between sociology, biology, and linguistics. The Introduction delimits the scope of linguistic (or grammatical) gender, which is an inherent morphosyntactic feature of nouns in about half of the world’s languages, introducing the definitions and notions which the present work utilizes to investigate gender. While focusing on grammar, this study has implications far beyond (e.g. for gender studies), and capitalizes on findings from other disciplines, such as cognitive neuropsychology. The chapter introduces the basic aim of the monograph, which intends to account for the steps through which the Latin three-gender system was reshaped into the binary systems shared today by most standard Romance languages (Spanish, Portuguese, Catalan, French, and Italian). One crucial definitional tool, highlighted in this chapter, is the distinction between target and controller genders: the two need not coincide everywhere, and mismatches between the two may arise—and did arise in Romance—through change.