In recent years the trading landscape was rapidly changing. Induced by advances in information and telecommunication technology and regulatory changes, two trends can be seen. On the one hand, the number of traditional exchanges exhibits a downward trend as effect of consolidation activity. On the other hand, the proliferation of alternative trading systems (ATSs) is rapidly progressing. The aim of this book is to review the existing literature on this issue, and evaluate the benefits and drawbacks of these developments in the trading environment on the micro and macro level. The micro level is the potential advantages and disadvantages for individual traders (especially institutional investors) and dealers as well as markets. On the macro level, especially, the implication on welfare and the efficiency of markets are of importance.