Final report on the Late Bronze Age and Early Iron Age evidence (pottery, metalwork, terracottas, architecture and other constructions) from excavations conducted by the University of Chicago at the sanctuary of Poseidon at Isthmia between 1952 and 1989. Stylistic analysis of artifacts offers important new information on Corinthian production: Isthmia has produced the first substantial collection of Early Iron Age Corinthian terracottas, for example, as well as 8th-century human figure depictions. Functional analysis, developing established methodology for site characterization, distinguishes Late Bronze Age settlement from Early Iron Age cult activity. Thus Isthmia may be counted among the growing number of Greek shrines established during the Bronze Age/Iron Age transition, and the nature and variety of cult practices at the site may be compared with those elsewhere. In its Corinthian context, Isthmia offers unique insights into 800 years of development, from Mycenaean province to Archaic polis.