The PeriodO project proposes a solution to this problem through the creation of a gazetteer that documents specific sets of statements made about periods by authoritative sources (“assertions”), rather than globally-recognized period concepts. This gazetteer will allow historical and archaeological periods to be integrated more fully into the Linked Data ecosystem. Periods have been very difficult to model as Linked Data, since there is no middle ground between the very broad period definitions provided by existing general thesauri and controlled vocabularies and the very specific chronological and spatial attributes associated with periods as they are used in individual datasets and publications. General period thesauri do not correspond to the period definitions used by many individual researchers, projects, or institutions. On the other hand, there is currently no way to reconcile the very specific period vocabularies deployed in different individual datasets with each other, in order to allow a search across those datasets to return all records associated with a date range of interest or with the same general period concept. The PeriodO gazetteer of period assertions will provide the middle ground between general and specific period vocabularies. Assertions in the gazetteer are modeled in a way that corresponds closely to traditional scholarly practice, which differs significantly from the use of controlled vocabularies favored by libraries and archives. Careful scholarship will generally qualify the use of a period term like “the Early Iron Age” with either a spatio-temporal definition (“that is, the period between roughly 1100 and 750 BCE in the Aegean area”) or a reference to a previous study (“as defined by Mazarakis Ainian [1997]”). A period assertion therefore consists of a statement by an authoritative source, in print or online, that associates a period name or term (“Archaic”) with chronological coordinates of greater or less precision (“700-480 BCE”, “late eighth to early fifth century BC”) and with particular modern geographic boundaries (“Greece”, “Turkey”, “Sicily”, etc.) or ancient or modern sites (“Athens”). Assertions will be recorded in their original languages. Each will be assigned a URI in the form of an HTTP Digital Object Identifier (DOI) minted by the EZID system of the California Digital Library.