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This paper provides a quantitative picture of structural change in innovative activities in four leading sectors — chemicals, pharmaceuticals, electronics and machinery — in Europe, United States and Japan and suggests an interpretative framework for explaining the differential rates of innovation among and within these sectors. This framework claims that an in-depth understanding of sectoral uneven innovativeness needs a bottom-up analysis of the technological, institutional and microeconomic characteristics of the different industries. The concept of sectoral system is proposed and applied for the four sectors. The conclusion of this paper is that structural change is guided by the specific coevolution of the variables of the sectoral systems: knowledge and technology, firms, networks and institutions.