The paper gives an overview of key themes of twentieth-century philosophical treatment of the language of science, with special emphasis on the meaning variance of scientific terms and the comparison of alternative theories. These themes are dealt with via discussion of the topics of: (a) the logical positivist principle of verifiability and the problem of the meaning of theoretical terms, (b) the post-positivist thesis of semantic incommensurability; and (c) the scientific realist response to incommensurability based on the causal theory of reference.