(en) Innovation is today at the centre of competition in most industries. As a stylized fact, an important channel through which engage in non-price competition is research and development (R&D), whose primary outcome is pure innovation. Firms carry out both product and process innovation, but the bearings of these two factors on R&D efforts have been investigated separately. Most of the rather large theoretical literature on R&D has focused upon process innovation, while product innovation has drawn particular attention in empirical studies. Recently, two different streams of research have emerged: one has contributed to the analysis of complementarities within R&D portfolios in monopoly or oligopoly models; the other has described the dynamics of adoption of product and process innovation and is commonly known as "technology life cycle" of the product. The aim of the present book is to bridge the two aforementioned streams of research by combining the theory of supermodular games and the theory of differential games. Starting from simple static monopoly models where we show that process and product innovation are complementary, we arrive at more complex dynamic oligopoly games where the presence of complementarity becomes an inherent feature of dynamic optimization. In particular, we investigate the technology life cycle and the issue of strategic interaction in differential games. Moreover, we study financial contracting in presence of complementary investment activities and different aspects that characterize advertising efforts.