Legacy system refers to an existing system that has been developed in the past and running critical business processes within a company. It could also be considered as the core of the company's system architecture that has significant business value. Therefore this valuable asset must be highly considered in the adaption to SOA. However, not in all cases the company can migrate the legacy system to SOA successfully. The migration process depends on some factors which vary from one legacy infrastructure and series of business processes to another. There is no easy way to transform the existing legacy assets. The fact that considering the right factors to reach legacy system migration success in a specific company is the key value. Therefore, we studied these factors from both business and technical perspectives. In this book, we studied and shared experiences about the factors influencing success of migrating the legacy system into SOA in five different case companies with different expertise including bank, airline, engineering and furniture companies. This book should help the researchers or the readers who are interested in the service-oriented architecture system.