This book is a collection of theoretical essays in the area of horizontal M&As and R&D with spillovers. It is divided into two parts, which respectively focus on the following issues. Part I:(competitive effects of external growth strategies)Horizontal M&As generate cost variation via uncertainty (Chapter 1) or via technological transfer (Chapter 2). We aim to study different types of horizontal M&A and find out which one is the most profitable from the national perspective, and to verify whether the M&A entry option is more efficient compared to others, such as Greenfield FDI and exporting, from the international perspective. Part II:(competitive effects of internal growth strategies)R&D effort contributes to cost reduction (Chapter 3) or to quality enhancement (Chapter 4), and it can be beneficial to other firms at no cost due to spillover effect. By considering the long-run R&D decision and the short-run price (or quantity) decision, we attempt to distinguish between full and partial regimes in terms of collusion or delegation, and to illustrate whether firms have incentive to adopt partial regime.