The present article investigates the functions of the enclitic particle =ci in Old Persian. It will be demonstrated that =ci is a focus particle that is polyfunctional. It functions as an additive particle and as an identifier. Moreover, it probably has an intensificatory function and can mark exhaustive focus. It will be argued that also in the forms yaci and aci, which have been analyzed as separate lexemes by some authors, the employment of =ci as a focus particle is observable. Subsequently, the involvement of =ci in the formation of indefinite pronouns will be discussed. It will be shown that, from a typological perspective, this involvement is inconspicuous given that, among others, =ci functions as an additive focus particle. During the treatment of the additive function of =ci, it will also be argued that =ci can fulfill the function of a discourse marker, which is also paralleled by additive particles in other languages. Moreover, it will be argued that =ci alone does not have an indefinite function, contrary to claims by several authors.