The objective of the present paper is to introduce a new approach to the study of the logical structure of scientific theories. We shall consider, in broad outline, a basic framework for handling theories and for analyzing a number of key metatheoretical concepts. The framework itself may be formally captured within a suitable system of set theory. However, its major innovative and heuristic thrust derives from some notions em ployed in modern abstract logic; in particular, it encourages the application of model-theoretic concepts and results to questions con cerning the structure and dynamics of empirical theories. The viewpoint adopted here shares some features in common with previous attempts at framing metascientific concepts, especially with the 'logistic' or model-theoretic, and with the so-called structuralist ap proaches. Like the former, it stresses the use of formal semantics in metascience; with the latter, it also appeals to some of the more abstract and structural characteristics of theories. However, it differs from these and other traditional approaches, notably in the role in which logic is portrayed. The nature and function of logic in scientific theorizing is here construed in an unusually liberal fashion: it is neither seen as a constraint on the syntactical form of theories, nor viewed as imposing limitations on the kinds of inferences permitted in science. This flexible and versatile conception of logic will be seen to amount to a somewhat radical departure from traditional usage. It also constitutes what the authors believe to be the clearest grounds for upholding the present framework. It should be mentioned at the outset that no new formal results are brought in this paper;1 nor do we pretend to offer a complete and polished semantical machinery whose fine details are fully developed at each point. Instead, we shall attempt to present something of the flavour of this framework with a minimum of technical fuss.2 We shall also try to provide some arguments in its support, and to indicate how it may lead to some fresh insights on a range of familiar, and sometimes controversial,