Systems integration has been an important topic ever since businesses started using mainframes to run their back-office operations. These systems specialized in common tasks found in functional areas such as accounts receivable and accounts payable, inventory, purchasing, and ordering. However, getting information from these fragmented systems to get a whole picture view of the business was extremely difficult if not impossible. By integrating back-office operations into a single system, enterprise resource planning (ERP) was supposed to solve that problem. In reality, however, very few companies fully implement all the modules of an ERP package and most continue to rely on legacy systems and other specialized software for their processing needs Integration of ERP with various enterprise applications remains a challenge. Moreover, it is not uncommon to have ERP software from different vendors or multiple copies of ERP software from the same vendor running in the same company. Integrating multiple instances of ERP software will be the predominant ERP project in most large corporations in the next half decade. Finally, many companies pursue merger and acquisition as a major growth strategy. A critical task in merging two companies nowadays involves integrating their ERP systems. This paper addresses these integration issues involving ERP systems. Common tools for integration and success factors for integration projects are discussed.