The research examines what drives Mergers and Acquisitions (M&As) using a theoretical and empirical approach. The theoretical part uses flexible optimal policies which adjust to changes in the market structure following a merger. The empirical part tests the major theoretical predictions to identify determinants of M&As in advanced economies. Chapters 1 and 2 consider M&As among firms in a pollution-intensive sector. Chapter 1 shows that identical polluting firms engage in M&As only if environmental policies are flexible. Chapter 2 shows that the flexibility of environmental policy increases the incentive to merge among heterogeneous firms. In addition, with flexible policy highly polluting firms have the highest incentive to merge than less polluting firms in a given sector. The empirical evidence suggests that the decision of manufacturing firms to engage in M&As is affected by environmental policy and firms may engage in merger deals in anticipation of a change in policy. Chapter 3 shows that with a flexible consumption tax firms in a bigger, more efficient country takeover firms in a smaller, less efficient country. The incentive to merge increases with the efficiency and market size of the host country. The empirical result obtained from 7 OECD countries shows that market size and firm efficiency play a major role in triggering international mergers.