Unified science is a recurring theme in Carnap's work from the time of the Aufbau until the end of the 1930's. The theme is not constant, but knows several variations. I shall extract three quite precise formulations of the thesis of unified science from Carnap's work during this period: from the Aufbau, from Carnap's so-called syntactic period, and from Testability and Meaning and related papers. My main objective is to explain these formulations and to discuss their relation, both to each other and to other aspects of Carnap's work.