The international literature has so far investigated many aspects of corporate diversification, with a recent particular emphasis upon firms' moves into new products (business diversification) and new technological fields (technological diversification). Indeed, it has recently been pointed out (e.g. Granstrand et al., 1997) that large firms are typically multi-technology as well as multi-product, and that therefore, careful attention needs to be devoted to the distinctions between products that the firm develops and produces, and the firm-specific technological knowledge that underlie its ability to do so (Pavitt, 1998) since the two follow different patterns. The purpose of the present paper is twofold: (i) it aims to investigate the largest firms' patterns of technological and product diversification over time, trying to highlight differences between European firms and their US and Japanese competitors, and (ii) it suggests a measure (based on the analytical framework developed in Teece et al., 1994) of the concordance between products and technologies which derives from and partly explains firms' diversification strategies. The data employed refer to a large cross-firm panel of technological and business activity of 248 large firms in the period 1977-1995.