Age of Acquisition (AoA) estimates match words or word phrases to the age at which children typically learn them: a word like mama will have a lower AoA estimate than nucleotide. This chapter describes objective and subjective approaches used to operationalize AoA in research on language learning and processing, discussing pros and cons of each, as well as addressing issues of reliability and validity of different constructs. The chapter concludes with a set of basic recommendations on which resources to prefer in different research set-ups.