 The Vienna Circle was a group of early twentieth-century philosophers who sought to reconceptualize empiricism by means of their interpretation of then recent advances in the physical and formal sciences. Their radically anti-metaphysical stance was supported by an empiricist criterion of meaningfulness and a broadly logicist conception of mathematics. They denied that any principle or claim was synthetic a priori. Moreover, they sought to account for the presuppositions of scientific theories by regimenting such theories within a logical framework so that the important role played by conventions, either in the form of definitions or of other analytical framework principles, became evident. The Vienna Circle’s theories were constantly changing. In spite (or perhaps because) of this, they helped to provide the blueprint for analytical philosophy of science as meta-theory—a “second-order” reflection of “first-order” sciences. While the Vienna Circle’s early form of logical empiricism (or logical positivism or neopositivism: these labels will be used interchangeably here) no longer represents an active research program, recent history of philosophy of science has unearthed much previously neglected variety and depth in the doctrines of the Circle’s protagonists, some of whose positions retain relevance for contemporary analytical philosophy.