My examination and rejection of Stegmüller’s rationality thesis, encapsulated in his characterisation SR of scientific revolutions, led me into an analysis of various aspects of the structuralist view of theories in what might be called its first phase of development; that is, up to about the mid-1970s. Since that time, the structuralist programme has gathered remarkable momentum. On the one hand, it has generated many new case studies of theories and theory change in science. On the other hand, its underlying metascientific framework has undergone further enlargement and modification. In these more recent developments of the structuralist view, the problem of incommensurability too has come in for increasing attention. Whereas in the first phase incommensurability was treated as a more or less imprecise notion belonging to the old, statement view of science (as in SR), in its subsequent stages of evolution the structuralist approach has attempted to force even incommensurability into its set of rigorously explicated concepts. Early efforts in this direction, located in Balzer (1979) and Stegmüller (1979), were none too perspicuous; and our discussion will, I think, profit little by dwelling on them. But a fresh attack on the problem, in recent works by Wolgang Balzer (1985b), and by Stegmüller himself (1986), appears to be somewhat clearer and better motivated. The present chapter will be devoted to it.