As the consequences of science and technology impinge more and more on international relations, states attempt to deal with the promise and dangers implicit in these consequences by the creation of international regimes. The nature and scope of such regimes are justified, in part, by the scientific and technological givens with which they are to deal. Increasingly, however, the process of justification is dominated by constructs and arguments taken from systems theory, thus mixing the epistemological styles of the natural and the social sciences. It is often not clear whether justification in terms of systems theory is rhetoric or based on demonstrated isomorphisms. The article seeks to answer this question by presenting a four-fold typology of systems theories together with their assumptions and relevance to the creation of international regimes. The article then examines three specific proposals for international action on science and technology, prepared under OECD auspices, in order to demonstrate the extent to which they rely on systems theory and to determine how persuasive the systemic justification is. The conclusion: there is an inverse relationship between the elaborateness of the systemic justification and the acceptability of the regime on logical, empirical, and moral grounds.